You can use rekordbox or Serato DJ Pro while your computer is connected to the DDJ-FLX6.
To use all the features of rekordbox or Serato DJ Pro, you'll need to subscribe a relevant rekordbox plan or purchase Serato DJ Pro Expansion Pack.
See [here] for the features available through Hardware Unlock in rekordbox.
■Serato DJ Pro
You can use Serato Pitch 'n Time DJ and Serato FX.
For Serato DJ Pro, see [here].