Fields in the _resource_ table

[My LinuxHAList]

>From https://cwiki.apache.org/confluence/display/VCL/Create+a+Linux+Base+Image

There is a step to do the following:

----
INSERT INTO `vcl`.`resource` (
`id` ,
`resourcetypeid` ,
`subid`
)
VALUES (
NULL , '13', '8'
);
------

Two questions:
1) The resourcetypeid=13, because this is an image, hence the value of 13.
    What about the subid ? What does "8" signify ?
2) After doing the insert, and say the id inserted is 13.
    Where is the resource.id subsequently being used ?
    [ is resource.id being referenced in _reservation_ or _request_ tables ] ?

Thanks