Optional

Optional is a term used in GURPS to emphasize GMs' ability to choose certain rules to tailor a campaign: ultimately any rule can be modified/ignored/added but it usually means it's not viewed as being a core rule.
==Mandatory optional rules==
In some cases it is actually required to choose one of two or more optional rules to have a system work: it involves taking a certain policy of application.
One example of that is the Paths and Books system on page 123 of GURPS Thaumatology. It notes:
:The GM decides which of the following optional rules apply.
Clearly one must choose either Mandatory Magery or Limited Non-Mage Ceremonies: there is no third option of choosing neither since it involves deciding whether those lacking Magery (Path/Book) are able to perform this type of magic.
This is unlike Magery Adds to Rituals or Magery Caps Skills: the third option exists to apply neither of them and use the usual policy of magery boosting the core skill of Thamautology.
Category:Rules