Wireshark · Wireshark-dev: [Wireshark-dev] what is the meaning of function proto_register_subtree_array?

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when you register a protocol,you should do the following three steps,

1.proto_register_protocol();

2.proto_register_field_array();

3.proto_register_subtree_array()

what does the third function proto_register_subtree_array do?why ?and do it for what reason?