sfz technical(ish) questions

1). Velocity is a linear scale: 0-127. Your ears perceive loudness in a (roughly) logarithmic way. So velocity split points do depend on personal taste, the core samples that you're using, and how many of them you use. But it's very often not as cut-and dried as 'half and half'.

2). Group= is an organizational tool. Whatever happens with this exclusive group won't affect any other groups, or anything else that isn't designated by a group. I'll use the first DimPro .sfz file that you'll run into as an example here. [Accoustic bd & sd.sfz under 02 - Drums]. Scroll down to the bottom for the choke groups. You'll see:

<region> sample=Accoustic Bd & Sd\closed hat 01.wav key=c7 group=1
<region> sample=Accoustic Bd & Sd\closed hat 02.wav key=c#7 group=1
<region> sample=Accoustic Bd & Sd\open hat 01.wav key=d7 off_by=1
<region> sample=Accoustic Bd & Sd\open hat 02.wav key=d#7 off_by=1
<region> sample=Accoustic Bd & Sd\crash.wav key=f7

Change the second line above to group=2. Now the closed hat 02.wav at c#7 will no longer choke the open hats below it (with off_by=1).

So let's choke the crash.wav with it instead, by adding off_by=2:

<region> sample=Accoustic Bd & Sd\closed hat 01.wav key=c7 group=1
<region> sample=Accoustic Bd & Sd\closed hat 02.wav key=c#7 group=2
<region> sample=Accoustic Bd & Sd\open hat 01.wav key=d7 off_by=1
<region> sample=Accoustic Bd & Sd\open hat 02.wav key=d#7 off_by=1
<region> sample=Accoustic Bd & Sd\crash.wav key=f7 off_by=2

It's a convenient way to keep one sample affecting only those samples that you choose, and separately from any other choke groups that you may want to set up. Notice that this group= has nothing to do with the <group> opcode at the top of this file. That's a way to group parameters across all <region>s that follow (until it runs into another <group> opcode). In this particular case, it sets all of the <region>s in the file to one shot mode.