What does it do?

This is an addon for FTB Teams that adds two simple configurable flags:

Allow PvP between Team Members (Default: False)
Allow PvP between Allies (Default: False)

These can be set per-team.

This PvP check integrates into:

The vanilla isAllied check itself, to properly handle targeting logic for tamed pets, summons, or similar mobs not targeting teammates or their pets
The vanilla damage pipeline, to prevent any actual damage or attacks from hitting teammates or their pets

These both also apply to owned pets, with an extra failsafe against infinite recursion, just in case a mod is doing something strange.

It has basically no performance impact, and is probably only required on the server-side, but works client-side as well, should any mod be checking isAllied there.

Doesn't FTB Teams already have something like this?

That confused me too. And maybe everyone else. No, it doesn't.

But FTB Chunks adds a team flag that disables all PvP within your team's claimed chunks.

Many players see that and think it applies to team PvP generally - it doesn't. That flag has no effect for anyone outside of the claim, and affects all players in your claim, not just team members. Hence this addon!