Tim Kurkoski and Todd Kopriva discuss Output module constraints in After Effects CS5:
'Now After Effects [CS5] knows the details of the constraints. When you choose an output module and change settings in the Output Module Settings dialog box, the settings are checked against this set of constraints. If there's a mismatch, a "Settings mismatch" alert and a yellow triangle icon appear at the bottom of the Output Module Settings dialog box. If you click the yellow triangle icon, a message explains what's going on...'
...read the details at After Effects region of interest. Note that some constraints like frame size are instead seen immediately in Format Options.
Update: see also Tim Kurkoski's answer in Another question regarding CS5 format constraints.