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When specifying markup, we can specify what markup will have a particular effect, and what markup will not have any effect. If we say nothing at all, that markup may have an effect in future releases. The specification is "extensible by omission" because we haven't specified everything. Some things have been left undefined to be defined later.
Example: In HTML, em and strong elements can be nested. If we just specify that **foo** will result in bold text, and //bar// will result in italic text, and we don't specify whether these will nest or not, then we are extensible by omission. Obviously they don't have to nest right now, but they might have to nest in a later revision of the specification.
Having spent an evening making my parser accept "badly" nested format, this is now part of "my" creole specification, which to be honest has had to make so many assumptions about what is "creole" and what isn't "creole", that I'm beginning to think I can't really call it "creole" at all because it neither is compatible, nor does "creole" provide any sense of being a specification with a common understanding of what it means.