Existem regras e há exceções.
man bash | grep -C 3 '&-'
Each redirection that may be preceded by a file descriptor number may
instead be preceded by a word of the form {varname}. In this case, for
each redirection operator except >&- and <&-, the shell will allocate a
file descriptor greater than or equal to 10 and assign it to varname.
If >&- or <&- is preceded by {varname}, the value of varname defines
the file descriptor to close.