Hello and welcome to the forum. Glad to have you here!
Most likely you are hearing members talk about rebuildable coils for an atomizer.
RBA: ReBuildable Atomizer (drippers or
tanks)
RDA: Rebuildable Dripping Atomizer (no tank)
RTA: Rebuildable Tank Atomizer
RTDA: Rebuildable Tank Dripping Atomizer
BF RDA: Bottom Feeding Rebuildable Dripping Atomizer
Macro, micro, nano all refer to the size (diameter) of the coil.
Nano coil - <1.4 mm in diameter
Micro coil - 1.4-1.65 mm in diameter
Standard coil - >1.65 mm in diameter