External fund numbers are typically used for by organizations that have a lot of different areas of support, and are a feature found in all of GiveGab's fundraising options.
As an example of how they are used, a university may have a College of Humanities and Sciences, which could have 50 different support areas or funds, such as History, English, Creative Writing, etc. In this instance, external fund numbers would be used to make reporting a little easier on the administrators so that they can ensure all of the funds that are made to each support area are being routed correctly. In essence, using this would add a specific number or code to each gift that comes in through that fundraising effort which would then display on your reporting page. If your organization does not need to use this feature for reconciliation purposes, you are safe to ignore it!