Faxes (Force Auxiliary) provide capital reps to both capital and subcapital friendlies alike. Fleet fax (buffer fit) are used when the enemy are expected to bring capitals while master fax are used when the enemy is expected to bring subcaps or only a few dreads. Typically the master fax are used if they can tank what the enemy is likely to bring, typically subcap vs subcap fights, while fleet faxes jump in to rep against capitals.
The Triage Module is the defining siege module of the Fax. Triaging vastly improves your remote reps, remote cap transfer and local active reps.
However triaging immobilizes1) ) your ship for the cycle duration (5 min) and prevents you from receiving remote reps or cap transfer. FC will tell you when to triage (triage green) and when to end it (triage red), getting this wrong can result in you being unable to jump away with the fleet and being left to die.
You will probably be jumping in together with carriers, supercarriers or titans. Watch broadcast for reps and capacitor. Your job is to keep them alive and provide capacitor against enemy neuts.
When you are out of cap and your triage cycle ends, broadcast for capacitor and a friendly fax2) ) , will top you up. When you are full go back into siege (this cuts off the capacitor transfer to you) and continue repping people.
If you are primaried however you are probably going to die as you cannot receive reps while triaged. Overheat hardeners, just try to last as long as you can before dying3) ) .
This differs from fleet fax in that usually only a handful of master fax are jumped in together. The skill requirements for flying this fit are higher and you have more responsibilities.
You will probably take turns going into triage to rep subcaps and/or other master fax, you will also need to take turns capping each other up.