"Andrew Gideon" wrote...

Responsibility for separation
between the separate elements in the flight is on the formation leader.

How does that work? Is visual contact required, or can this be done
electronically?

In most cases, visual separation. However, when working in altitude blocks, as
with larger tanker/receiver formations, some of the separation may be done with
air-to-air radar. Even in that case, though, the visibility must be good enough
for visual rendezvous in close.