Hotend Low/High meaning:
By default the switching hotend has one low temp and one high temp side. The low temp. (250C) side is better for softer filaments like PLA and PVA whilst the high temp. (400C) side can handle filaments like polycarbonate and nylon.
If you opt for High/High you will receive hardware for creating two high temp. sides in addition to the low temp. hardware too.