clutch friction material
the rubber from all the bonded pistons in these
fine metal dust suspended in the fluid from worn hard parts.
If you let it slip shift too long, the clutch packs will have worn/overheated, no way to fix that except a full rebuild. Same thing for the bonded rubber on the pistons, if the fluid got too hot and any of the rubber failed, full rebuild.
If it were mine, I would just do a full valvebody replacement and see if that fixes the symptoms. I would also start looking around for a Focus being parted out on my area because equal chances I would need a $300-400 take-out transmission.