There are two types of custom symbols: subcircuit symbols and hierarchical symbols. What you are referring to is a hierarchical symbol that links to a schematic file (.qsch).
Refer to this post to see if it answers your question.
Subcircuit schematic designator - QSPICE - Qorvo Tech Forum
I have a complete study documented in Part 2 of this general guideline
General Reference Guide by KSKelvin.pdf - KSKelvin-Github