Noobie-doobie PDC Questions

[rbet]

I kinda understand what PDC is... some plugins take (relatively) a lot of time to process a signal; to compensate for this, PDC is required.

My questions are:

1. How is Plugin Delay measured? In seconds?

2. Why do only some plugins require PDC and others do not?

Can someone post an short audio example of a track with a plugin with PDC and then the same track with PDC disabled? I guess the VST effect kicks in late in the second part?

edit: one more:

3. Does it affect only VST effects or instruments also?

[rbet]

c'mon, I know you guys are better than this!