From D&D Wiki
This is an auxiliary template that allows one to encode "
||" (two pipes or vertical bars) within template parameters. If it is unnecessary to get a "
||", it is better to use "
||". However the "
||" delimiter for a Wiki table has to be escaped by a template if it is used within a template.
There is no such problem with "
!!" (exclamation marks). This problem only affects "
||" (vertical bars) in conjunction with the Parameter default mechanism within tables.
This template cannot be substituted.