When you choose an output module and change settings in the Output Module Settings dialog box, the settings are checked against a set of constraints for certain formats. For example, the MPEG-2 DVD format output only allows parameters that fit the DVD specification; movies created outside of these constraints may not play in a DVD player that conforms to the DVD specification.
Many formats are unconstrained. Many codecs used inside QuickTime and AVI containers are unconstrained, as are all of the image-sequence formats.
In After Effects CS5, there is a known issue that causes WMV and FLV output frame rates to be constrained unnecessarily.
The frame rate values for WMV should be unconstrained, but After Effects CS5 constrains the frame rate in WMV files to a preset group of values: 1, 5, 6, 7.5, 8, 10, 12, 12.5, 15, 24, 25, 29.97, 30, 50, 59.94, 60.
The frame rate values for FLV should be unconstrained, but After Effects CS5 constrains the frame rate in FLV files to a preset group of values: 1, 5, 8, 10, 12, 12.5, 15, 24, 25, 29.97, 30.
By creating an output module template and applying this template to a render item, you bypass this bug. Because a template isn't associated with a specific composition, there is no associated frame rate for it to check against, so the constraint checking doesn't occur.
For more information about frame rate, see "Frame rate".