A few specifics:
- Power Flat Flat Wound is available in 4-, 5- and 6-string.
- Silencer Nickel Semi-Flat 5- and 6-string sets are well suited to fretless basses because of their smoother wrap surface, though a standard roundwound set still works on fretless.
- Multi-scale sets (Power Wound and Power Steel) come in 4- and 5-string only.
Check that each string's winding length and taper location fit your bass, especially on extended-range basses.