When using contact mode with a boost cable, it doubles the voltage which quadruples the power. This can be a good thing depending on what you are trying to do.
However, it does require that you know how settings affect this mode of operation since you are combining the signals from out 1 and out 2. Most of the time this is done via inverse+sync.
If you wish to apply an offset, since the signal is inverted on out2, you need to use -100 for the offset here so that when it's inverted it's 100.
In other words, if you use 0/0 you are good to go. If you use 100/100 the signals will cancel each other out and you are left with nothing. If you use 100/-100 then the end result will be a full positive only waveform.
That might sound like a lot to consider, so to start I suggest the following when using the boost and contact modes.
Stick to inverse+sync until you feel comfortable on what's going on with the two outs and how they signals interact.
Stick to 0/0 for the offsets when healing.
Select 0/0 or 100/-100 for the offsets when killing.
Always check reduce amplitude to be safe.
This will be a great starting point.
As for when 100/100 is useful with the boost, it's for advanced signal generation that doesn't use inverse+sync.