What exactly is "breaking in" on a new cartridge ?

Stylus tip ?? Suspension ?? Or......
Suspension, coils all moving parts developing a working motion together. I find that my Kleos now well "Broke-in" still sounds better once I play one LP side to "warm up" for each session. YMMV
me too theo - my cartridge makes the biggest improvement when it is warmed up.
What they said, plus your ears and brain.
I am breaking in a Ortofon 2M Black
Those (burn-in, break -in) are frightening expressions or phrases considering the fragile nature of the objects involved. In Europe we use the expressions 'play in' or 'warm-up'(time) which are obviously more adeqate for the purpose.

New piston rings, new semi-auto handgun, etc., these
things need mechanical "wearing-in", so they loosen up and
operate with less friction.

What is happening to a brand new cartridge during it's
"break-in" period ? I do not mean "warming up" for
regular daily playing.
Noslepums, if you read my first response, I indicated that all moving parts within the cartridge need to wear in so that they move freely in interacting with each other. I only indicated that a "warm up" period seems to help for each session. But that was not in place of the "break in" period. There are elastimers that hold the coils and stylus
and they are more "stiff or ridgid" when new. So as the record plays those parts "limber up" to move more freely and establish a path of movement that it will assume each time it plays. I think you answered your own question with you last comment. Yes as a piston ring wears to fit the cylinder or the action in a gun wears to slide more easily, a cantilever to which the coils are mounted need to do the same. The suspension elastimers need to limber up to reduce resistance to movement and more accurately track the groove. It has to move not only up and down but side to side as well. An analogy offered is a blind man reading braille with a limber finger rather than a splinted stiff finger.
this cross section may give you an idea of the inner parts
involved that need to "break in".
actually maybe this is a better read.
It would be interesting to see a graph indicating the progression of change in cantilever suspension elasticity (or stiffness) over time played. As the suspension is stressed from movement of the cantilever and as the suspension becomes less/more elastic with age, the response of the cantilever and stylus changes. The ability of the stylus to trace the groove laterally and vertically, the quality of pickup (cantilever vibration control) and degree of resonance damping are affected during this protracted "break-in" period. Thus, "break-in" is ongoing and continues until the cantilever assembly is either rebuilt/replaced or the cartridge is tossed in the trash.

But is the relationship linear or curvilinear? There may be some validity in the idea of "settling-in" if the relationship is curvilinear; the initial period of "break-in" may be a rapid progression until a plateau of relative stability is reached. If this is the case then there is merit in the idea of a cartridge needing to "settle-in" until it's optimal level of performance is reached. Depending on the type of cantilever suspension that period could take more or less time. And the stresses that the cantilever are put through (range of frequencies and dynamics) as vinyl is played will likely determine the "settling-in" period.

First, Thank you to Theo and all others who have helped
shed some light on this topic.

Still though, I'm having a hard time digesting parts
of this.

I hope not to come across as arguementative or confrontational, but I have more questions. I must know !

The diamond stylus, I have been told, does not appreciably

"...moving parts developing a working motion together..."
I can only understand moving parts "breaking in" that
either contact each other, or deform (plasticly).
This would leave out all the coils, magnets, and wires.

The rubber cantilever suspension, OK, I can see that possibly.
I did not know the coils/magnets were mounted on rubber
(that doesn't seem like a good idea, but I don't know).
But I haven't heard of rubber/plastic parts in other applications needing a compliance settling-in period.
In general, rubber only hardens. But, I could well be
ignorant here.
Metallic coil springs, for example, start out with 100%
of their spring constant and it's only downhill from there
with use.

"...plus your ears and brain."
Hmm. Is this like becoming re-acclimated to a periodic
activity, or sliding into a mood, or...? Interesting.
Sounds more like "warm-up". No questions from me here.

If "...break-in is ongoing..."
If break-in is ongoing, is this not simply "wear" ?

Anyway, thanks again gents.
Neslepums; my comment on "break-in" being an ongoing phenomenon was somewhat cynical. :-) You're correct; "break-in" as used with phono cartridges is really just wear. But as I suggested, if the suspension does have a wear rate that is rapid at first and then settles in to a gradual wear rate then you might conclude that a cartridge needs to "break-in" before it settles in to it's typical state of operation and sound.

But I've always just installed the new cartridge, set a LP on the platter, and played music without paying attention to cartridge "break-in". It either sounds good or it doesn't.

Nosleplums, as you indicate and as does Tketcham, yes you are right. The break in period is the initial wear in period. But once the related parts wear in the rate of wear is dramatically reduced. No different than the rings against a cylinder other than there is no lubricant. As far as the stylus it doesn't really break in to fit the groove. It is harder than the vinyl and played repeatedly will wear the vinyl before any visible wear occurs to the stylus. Eventually, yes the stylus does wear and then you get to buy a new cartridge and start the process all over again. But hopefully that is many many hours of enjoyment later. Just think of it as new jeans or new shoes that need to flex before they are comfortable. The moving parts a just "limbering up" to work more freeley and as mentioned earlier, to more accurately follow each and every nuance of the record groove, extracting as much information as it can.