Are solahma, second line units, and provisional garrison clusters all synonymous?
I believe that they are not, and the confusion arises from the various writers interlacing the terms inappropriately. Each should be distinctive in nature but are not always written as such.
A second-line unit is a unit comprised of secondary personnel and equipment, but should be part of the Clan Touman as an integrated unit. However, the unit could be staffed by older warriors, e.g. solahma, but still be considered a more prestige’s posting than a provisional garrison cluster (PGC).
Consequently, a PGC, should be a temporary collection of equipment and personnel meant to meet immediate Touman needs in a combat/occupied zone. However, a PGC could be active so long, it is considered a second-line unit with the initial designation losing its meaning. Thus a PGC is where you should be expected to find more solahma, due to their low standing in the Clan Touman, but not exclusively.
Continuing, a solahma field-unit is likely an even more temporary unit than a PGC that the Clan Touman doesn’t even bother to name, because its only going to be around long enough to die gorily and gloriously for its Clan. Conversely, this unit should not qualify as a secondary formation or PGC.
Thus, the solahma term should always be a passing label as newer and younger warriors are always being cycled through the Clan Touman; even if those warriors are younger older warriors. So, the term solahma should be used more as a descriptor, rather than a permanent designation.
Though as stated earlier the writers do use the terms interchangeably, and maybe my head cannon is all wrong.