Technically, they are called upfitters. For the 05 and up you can get a switch panel for them from Ford that mounts to the lower dash. I think 2 are 10 amp relays and 2 are 30 amp relays.
To utilize for say lights, run ground to batt or frame, run power to proper upfitter, connect upfitter switch, and you now have lights with a relay.
I think he's talking about the ones under the hood that just 'dead end' on both side of the firewall. They are there to make it easier to bring wires into the cab (for connecting to upfitters etc)
2007 F350 SC FX4 Lariat
Edge Insight CS w/ Pillar Mount & EGT
SCT with Gearhead Tunes
ARP Studs & EGR Delete
Back Rack & Tool Box
LED bed & toolbox Lighting
4 Hella's Behind the Grille
18W LED Hitch-mount Back-up Lights
Dually ID Bar