There are a few exceptions, but in general the difference is in guaranteed transformation efficiency as follows:
Subcloning Efficiency cells are guaranteed to produce at least 1.0 x 10E6 transformants per µg of transformed pUC19 or pUC18 supercoiled plasmid
Library Efficiency cells are guaranteed to produce at least 1.0 x 10E8 transformants per µg pUC19 or pUC18 DNA
MAX Efficiency cells are guaranteed to produce at least 1.0 x 10E9 transformants per µg pUC19 or pUC18 DNA