Some phrases when team building.

Main - means this unit is the one constantly on field

Sub - means unit works to be off field

Main DPS - usually means centered around one unit to maximize their damage (e.g., Hutao vape team)

Sub DPS - unit that provides other forms of damage while off field (e.g., Fischl's Oz, Xiangling Burst)

Support - units that help main/sub DPS units (e.g., Bennett Burst)

These are just conventional team roles. More recent team comps have blurred the lines a bit. Like C0 Raiden, she's more a Main Burst DPS type, when her Burst is over, you want to quickly cycle through your subs to get their Skill/Bursts off and charge up Raiden's. Her normal attacks are DPS loss. Or, the taser team of Sucrose, XQ, Fischl, Beidou. Yeah, Sucrose is the one on field the most, but she's more like Main Support(?) A primarily to swirl and trigger everything. Unless you failed VV set stacked for DPS (not just getting by with 4 set VV bonus on mismatched stats).

Melt - means team built so order of reaction is apply cryo first and then trigger with pyro (i.e., 2x reaction damage), so EM it's better on pyro until

Reverse Melt - means team built so other of reaction is apply pyro first then trigger with cryo (i.e., 1.5x reaction damage), so EM is better on cryo unit