tag作用:
1.指示你正工作在那个branch上。(因为每个branch都有一个tag.
2.如果你不想更新一个大目录树的一部分,你可以利用sticky tag.
(sticky tag的定义:If you check out a certain revision (such as 1.4) it will become sticky. Subsequent cvs update commands will not retrieve the latest revision until you reset the tag with cvs update -A.)