Local citations are a lightweight alternative to full source citations — little more than a footnote. They are useful when the only reference is an index entry (such as a GRO index of births, deaths, or marriages) rather than a complete document.
Click the + button on the citations list and choose the "local" option. The Local Citation dialog appears, where you can enter free text or use the helper tabs (such as "GRO" and "Scots") to build a formatted entry. Click Build to generate the text, then OK to add it to the fact.