For example, you can type M-d , then insert another number. Editing the value does not actually set the option variable. To do that, you must set the option. Some options have a small fixed set of possible legitimate values. These options don't let you edit the value textually.
Some options have values with complex structure.
For example, the value of load-path is a list of directories. Each directory in the list appears on a separate line, and each line has several editable or active fields. You can edit any of the directory names. Two special commands, TAB and S- TAB , are useful for moving through the customization buffer. Typing RET on an editable field also moves forward, just like TAB .