HOW TO LOCK AN UNIT IN D2

In your paper doll, choose the lock option and click on the unit you want to lock. The action is irreversible and you won't be able to change you mind later. This allows to keep an unit at a certain level if you don't want to build the structure in the capital or if you prefer not to get the next upgrade.

It is better to do it before the next level of that unit so you won't loose the points for the experience if that unit if near its next level or ready to upgrade. If the unit is ready to upgrade, the little cross will just disappear. Your unit will continue to move up once its type has been locked. This feature is only available from D2 1.41 and above.

HOW TO DISMISS AN UNIT

You can only dismiss an unit, not a leader. In your paper doll, choose the dismiss option and click on the unit you want to dismiss. The unit will just disappear, you can buy another one later in a city or a capital. This option allows you to buy a better unit at a mercenary camp and you are far from a city or when you cannot give this unit to another friendly leader. Since you can not dismiss a leader, just send him to a city or get him kill somewhere.

If you don't want to use a leader or if you are done with him you can always use the skip option "guard" but you have to do it each turn. This option is handy when you have many leaders, and you need to go through them to give your commands. Once you are done with a leader just put him on "guard".