What is this symbol? A circle with an arrow joining two connections and another arrow pointing to it
Due Intellectual Property and business reasons, this company does not wish to disclose much details without an NDA.   It could be a Hall Effect device with current output due variable reluctance current of rolling train wheels or other magnetic sensor that amplifies current with a transistor.

Regardless it is an obfuscation of how the Current source works except input and output share a common sink unlike the Ebers Mols model in a transistor.

So it could be a CCCS or a VCCS. You want us to guess?