We have been told before that "cái ca" is mug, closer to a cup surely than a glass. A 'cup' of milk is really not interchangeable with a glass, whereas a mug is a closer description and choice for 'cup.'
"an orders a cup of milk" mean "Gọi một ly sữa"
Cup = Tách or Cốc; Glass = Ly; Mug = Ca.
An gọi một cốc sữa