Tag is a special parameter which can be given for a set of documents. The main purpose of tags is to join a number of documents into one group and then while doing search to select a group of documents to search through.
You can use Tag command of indexer.conf to give some tag value for a server or server subset. While doing search you can specify tag value to search through documents which tag matches given parameter with t=xxx parameter, which can be passed from HTML form. Take a look into Section 3.4.6.33, indexer.conf-dist and search.htm-dist for explanation and examples.
Tag type is CHAR. CHAR type allows to use some nice features. You can use '_' and '%' LIKE wildcards in tag parameter when doing search. It makes possible that tag, like a category, does support an idea of nesting. For example, documents with tag value "AB" can be found with both "A%" and "AB" tag limits.
Tags also give a way to make an URL a member of multiple tag selections. Playing with LIKE wildcards you can easily create two or more groups.
For example, tag "ABCDE" is the member of at least these selections:
_BCDE A_CDE AB_DE ABC_E ABCD_
Note: If you have big enough database and often use tag limits, it is useful to create an index by field "tag" in "server" table. This index is not created by default.
CREATE INDEX srv_tag ON "server" ("tag");By default, the length of tag field in url table is limited by 16 characters. If you need more, increase this length before DB creating.