That is hard to say as this depends on the type of UFL (or what SDK it was built with). COM UFLs need to be registered.
For UFLs created in VS .NET see KBA 1266912 - How to deploy Crystal Reports UFL created in Visual Studio .NET
And then there are UFLs that do not need anything other that being in the bin folder (same place as u2lcom.dll).
- Ludek