Per the docs:
type :: :atom ## Atoms | 1 ## Integers | 1..10 ## Integers from 1 to 10 | 1.0 ## Floats | <<>> ## Bitstrings | <<_ :: size>> # size is 0 or a positive integer | <<_ :: _ * unit>> # unit is an integer from 1 to 256 | <<_ :: size * unit>>
you can read about it here.
