If you look in the Elixir code, in the “inspect” module, you can see that the output is derived from calling Function.info. Calling Function.info on an anonymous function on my machine gives me:

iex(5)> some_function = fn x -> x * x end
#Function<7.126501267/1 in :erl_eval.expr/5>
iex(6)> Function.info(some_function)
[
  pid: #PID<0.109.0>,
  module: :erl_eval,
  new_index: 7,
  new_uniq: <<241, 72, 50, 109, 70, 84, 198, 45, 20, 94, 42, 25, 184, 243, 5,
    100>>,
  index: 7,
  uniq: 126501267,
  name: :"-expr/5-fun-4-",
  arity: 1,
  env: [
    {[], :none, :none,
     [
       {:clause, 5, [{:var, 5, :_x@1}], [],
        [{:op, 5, :*, {:var, 5, :_x@1}, {:var, 5, :_x@1}}]}
     ]}
  ],
  type: :local
]

So you can see that the inspect value for the function #Function<7.126501267/1 in :erl_eval.expr/5> consists of the function’s index, uniq and arity pieces.

According to the documentation on Function.info:

:index - (integer) an index into the module function table.

:uniq - (integer) a unique value for this function. This integer is
    calculated from the compiled code for the entire module.

:arity - (integer) the number of arguments the function is to be called
    with.

Now… having figured all that out, it’s still the case that there’s not much the average user can do with that information (well… the arity is useful metadata) - most folks can treat it as a blob of stuff that uniquely identifies the function.

It’s not double. The dot is just a separator. Similar to as in a PID individual segments have their own meaning, but which is not documented nor necessary to know.

What do you mean by “double”? Anyway, there is no meaning in that number. It is opaque., though the /2 indicates arity of the anonymous function.