What does it mean when an OCaml variable starts with `#`? (hash sign/pound sign)?

I cam across this in a pattern match

| {call_name = #bundle_source; _ }

source

Earlier in the code, bundle_source is defined as a type (type bundle_source = ...).

So what does the hash sign mean? Does {call_name = #bundle_source } in the pattern match mean that the value of call_name is expected to have type bundle_source?

I searched the manual for "hash sign" and "pound sign" but found nothing.