I can't seem to find anything concrete on this, and would like to know if I run into it in the future.

It works on all arcs. Any card effect works on all arcs unless it specifically spells out that it only works on a single type of arc.

And any printed arc is an arc. It doesn't matter if that arc is actually used for anything. If you get behind a naked ghost, you get fearlessness, even though they can't shoot anything out of that arc.

8 minutes ago, Zefirus said:

It works on all arcs. Any card effect works on all arcs unless it specifically spells out that it only works on a single type of arc.

And any printed arc is an arc. It doesn't matter if that arc is actually used for anything. If you get behind a naked ghost, you get fearlessness, even though they can't shoot anything out of that arc.

This is correct. It works on primary, special, mobile, auxiliary arcs, and any other sort of weird and wonderful arc that FFG should later deign to design.

On 6/8/2017 at 11:17 AM, Zefirus said:

It works on all arcs. Any card effect works on all arcs unless it specifically spells out that it only works on a single type of arc.

And any printed arc is an arc. It doesn't matter if that arc is actually used for anything. If you get behind a naked ghost, you get fearlessness, even though they can't shoot anything out of that arc.

One small caveat: the guide lines on the Lancer-class pursuit craft's base are just there to show you where your mobile arc can be. They do not create three extra always-present printed arcs. Only the section where the mobile arc indicator is pointing is an arc.

What about 360 arcs like Falcon, K-wing, or JM5k?

There's no such thing as a 360 arc. The ships you mentioned have a normal frontal firing arc, and the ability to shoot outside that firing arc via the Primary Weapon Turret rule. With these ships, Fearlessness works only within the printed frontal arc.