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Why is it necessary to choose a format from a limited list or to go to the trouble of trying to create a custom format?
Because only you know what you want to do with the final video. If you're going to burn it to disc, upload it to YouTube or use it in another video, those have very different encoding needs and there's no way that PD would "know" what pixel size and format you'd want to use for each project.
Once you've selected an output profile, PD should remember it as the default so if your source clips and output goals are the same with each project, you shouldn't need to make any changes when producing.
Starting in PD17, there's a tool on the Produce page called Profile Analyzer, which looks at the details of your timeline clips and will suggest (or even create) the best profile for you.
In PD16, you can see whether SVRT is available for your current project by typing
Alt+S or by right-clicking on a blank section of the timeline and choosing Show SVRT Track.
A green bar above the clip(s) means you can use SVRT, and you'll just need to write down the details shown in the
Custom Profile box that appears above the timeline. Then find the closest
Default profile on the Produce page and click on the
+ button to create the custom version and make the necessary changes.
Once you've created that (or used a default profile if one gives you the SVRT option when it's selected), PD will remember that until you change it.