MemSet[f[x_], body] is like f[x_] := f[x] = body, but depending on the value of the setting of FCMemoryAvailable -> memorycut (memorycut - MemoryInUse[]/10^6)
MemSet[f[x_], body]
f[x_] := f[x] = body
FCMemoryAvailable -> memorycut
memorycut
MemoryInUse[]/10^6
MemSet[f[x_], body] may evaluate as f[x_] := body.
f[x_] := body
Overview, FCMemoryAvailable.