what's the slot right before the volume one for?

[Darkware]

The first slot is the note, the second one is the instrument, the third I have no idea and have never seen it used, then there's the slot that has 0,1,2,3,4,5,6,7,8,9,a,b,c,d,e, or f in it and I can't figure out what that one does. The last one is for volume of course. I've tried to search for tutorials explaining the basics of this as I'm very new to this stuff. Is this slot for the effects?

Any links to helpful tutorials would be appreciated if you happen to have any as well.

[Saga Musix]

A-4 18 v05 SC2
A-4 is is the note. 18 is the instrument. "v" stands for "Volume", 05 is the parameter for v, "S" is a multi-effect and C2 is its parameter.
Thus, this would play the note A-4 of instrument 18 at volume 5 and cut it after two ticks (that's what SCx does, at least in the S3M/IT/MPTM format). You can double-click on any part of each cell to get a description what it does and change its value.

Note that the "volume column" (the one where the v05 is in my example) cannot be used in the MOD format, and various effects are not available in every format and are named differently in some formats (f.e. MOD only has effects 0 through F, XM has effects 0 through Z, S3M / IT / MPTM have A through Z).

[Rakib]

Just double click it to reveal what it does, actually it is a lot more than just volume.

Here is a help file you can look trough.
http://forum.openmpt.org/index.php?topic=4387.0
Look from page 166 for the actual commands for fouth slot.
Page 174 has the commands for the third slot.

Im guessing you are using IT or Mptm format.