Used for content that may or must be replaced by the user.

SELECT * FROM <replaceable>tbl_name</replaceable>

Here, the user is expected to replace tbl_name with the name of an actual table when issuing the SELECT statement. Pearson notes: All placeholders should be in mono and italics. A placeholder is pseudo text you put in code (a file name, or parameter) where the reader will need to fill in their actual info to complete.

http://docbook.org/tdg/en/html/replaceable.html