Optional conditional abilities are ones that use the word "may" in their effects. When an optional conditional ability's trigger condition is met, the owner of the ability gets to choose whether or not to trigger it. The opposite of an optional conditional ability is a required conditional ability.

Page 21, Column 2, Paragraph(s) 2, Core Rule Book
If a conditional ability uses the word "may" in its description, it is an optional conditional ability. The decision to trigger the ability belongs to the player who controls the card, provided the ability's trigger condition is met.
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