If you're talking about a basic power amp with a passive volume control on the input then wide-open relates to full volume. If you're talking about an integrated amplifier with and active input stage then the amp will probably be pushed to full volume/clipping well before the maximum volume setting is reached. The point at which you reach full volume will depend on the output voltage of your source and the recording level of the particular recordings being played...
Gain, volume and output power
In addition to the thread I recently started about what the difference is between slew rate and rise time, I would like to have some explanation about the following subject: what is the relationship between gain, volume and amount of power that an amp delivers. Let's say an amp is capable of delivering 200wpc into 8 Ohms. Is this with the volume knob wide open or doesn't it relate to the position of the volume knob?