Short answer: `HSK63-ER11` is a HSK63 machine-taper holder fitted with an ER11 collet chuck; it holds tools from 1 mm to 7 mm shank diameter. The HSK63 end goes into the spindle (1:10 hollow taper, DIN 69893, flange-contact), the ER11 end grips the tool.
Collet range is set by DIN 6499. A 12 mm end mill needs ER11 or larger; an ER11 will not close on a shank outside 1–7 mm.