The double colon (::) has two uses. Both could be subscribed as “being of type”:

1. In binaries, it is used to explain what size(bit/byte/multiple bytes) an element in the binary is. An example, taken from the Parsing UDP with Binary Pattern Matching blog post:

    <<
      _header        :: size(240), # 30 bytes * 8 = 240 bits
      priority_code  :: bitstring-size(8),
      agent_number   :: little-unsigned-integer-size(32),
      message        :: bitstring-size(320)
    >> = data

When pattern matching with binaries, a size has to be specified for all but the last element to-be-matched element.

2.In specifying what return type a function has, when writing @spec, @type or @callback attributes:

@spec fibonacci(int) :: int
@type num_or_str :: number | string
@callback read_description(Entity.t) :: iodata

Here it is in the docs.

It’s not an operator. Think of it as saying “is of size” So title
is-of-size 30 bits.

It looks like that is binary pattern matching, so the value after the double colon specifies the size of the value before it.