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

From: "??????????" <237825552@xxxxxx>
Date: Sun, 16 Feb 2014 20:44:36 +0800
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?