From Sega Retro
X3I is a new instrument bank format developed by Oerg866 for use in his xm3smps/oerg music conversion tool. It is the successor to X3V. It can contain up to 65535 YM2612 (X3V can just hold 255) instruments formatted according to the SMPS specifications; additionally, it can store a description for each instrument.
The internal structure of an X3I instrument bank is simple. The first three bytes spell "X3I", the next two bytes specify how many instruments there are in the bank (0 to 65535); after that, the actual instrument table begins, in the following format:
|25 bytes||0 to 255 bytes|
Note: The X3I format was created to eliminate the limit of just 255 instruments.