Plans in RedTeam Flex use one standard list of disciplines, so sheets are labeled the same way on every project. You can't add your own disciplines. To group sheets your own way, use tags.
Pick a discipline
When you review an upload, choose each sheet's Discipline from the standard list, and add Tags for anything the list doesn't cover.
To change the discipline on a sheet that isn't published yet, click the pencil icon next to it in the Plans list and choose from Discipline.
Types
Sheets no longer have a separate Type field. Plans, specs and addendums each have their own tab: Plans, Specs and Addendums. Specs don't have a discipline.
If you've done this before
You used to be able to create your own disciplines and types, in Configuration or with + Add new in the lists. Those options are gone: the Plans and Specifications settings are no longer in Configuration, and the Discipline list offers only the standard values.