what does the function "proto_register_subtree_array" used for in register of every protocol?

["??????????" <237825552 () qq com>]

hello,i wanna know what does the function "proto_register_subtree_array" used for in register of every protocol?eg:
in packet-ieee1722.c file:
   /* Register the protocol name and description */
    proto_1722 = proto_register_protocol("IEEE 1722 Protocol", "IEEE1722", "ieee1722");


    /* Required function calls to register the header fields and subtrees used */
    proto_register_field_array(proto_1722, hf, array_length(hf));
    proto_register_subtree_array(ett, array_length(ett));


and what does the ett used for?

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