It's called chaining and as you say is part of the building process, in particular a way to build comb over an open expanse.
Possibly the bottom bar 'festooning' is them building burr comb between the bottom bars of an upper box to the top bars of a lower box?
Possibly the bottom bar 'festooning' is them building burr comb between the bottom bars of an upper box to the top bars of a lower box?