The NinjaTracker format only allows 15 instruments.

Attention: Because of limitations of the Apple IIGS, looped instruments are split up into two instruments: The instrument and the loop. So a 15 instrument MOD can end up needing 17 instruments in NinjaTracker and is therefore not convertible.

The Apple IIGS only has 64k sound memory, and NinjaTracker has to fit all instruments in there. Even worse, instruments need to be placed at certain memory boundaries, which may reduce memory even further.

Also, at the end of every instruments, 8 stopper bytes are added.

This results in the following boundaries for instrument sizes:
248
504
1016
2040
4088
8184
16376
32760

So for instance, if a MOD instrument/loop header or loop is 2050 bytes, its boundary is 4088, and potentially wastes a lot of memory. In this case, it would be sensible to try and reduce the size to 2040 bytes.

The converter will give hints on how to tweak the MOD instruments.

NinjaTracker does not support all effects. Unsupported effects are ignored.

0 - Arpegiatto
1 - Portamento Up
2 - Portamento Down
3 - Tone Portamento
4 - Vibrato
5 - Tone Portamento + Volume Slide
6 - Vibrato + Volume Slide
7 - not supported (Tremolo)
8 - not supported
9 - not supported (Sample Offset)
A - Volume Slide
B - Position Jump
C - Set Volume
D - Patternbreak
E - Misc. Cmds, see below
F - Set Speed

E0 - not supported
E1 - Fineslide up
E2 - Fineslide down
E3 - not supported
E4 - not supported
E5 - not supported
E6 - not supported
E7 - not supported
E8 - not supported
E9 - not supported
EA - Fine Volume Up
EB - Fine Volume Down
EC - not supported
ED - not supported
EE - not supported
EF - not supported