The context would be helpful, as we could provide an answer that gives you some practical guidance.
"External jurisdiction" needs a contemplation of what jurisdiction is.
Jurisdiction is, broadly speaking, the limit or extent to which an authority has control over a particular subject. It could be referred to in varying ways, for example: monetary amount, territory (state/federal), and area of law, are the most common definers of jurisdiction.
A practical example is that the NSW Local Court has jurisdiction for civil disputes in claims of up to $100,000. Civil claims for more than $100,000 are outside its jurisdiction. But it also only has jurisdiction for matters which arise in or are connected with New South Wales. It couldn't, for example and in simple terms, determine a civil matter of $50,000 if all relevant aspects of the matter happened in another state.
An 'external jurisdiction' generally means a reference by a court in a particular jurisdiction talking about a matter which sits outside its sphere of influence (i.e. outside its jurisdiction). That's not the way it's commonly expressed in all situations, however. In Australia, a state court could refer to the 'Federal jurisdiction' as such, instead of as an 'external jurisdiction' - which it technically is.
The phrase 'external jurisdiction' is generally reserved for situations where a range of possible jurisdictions could be referred to without knowing which one in particular. It could be used where a court in this jurisdiction has the power to call on a court in another jurisdiction. An example of this is the Bankruptcy Act which allows the courts to call on the aid of courts in other ('external') jurisdictions for assistance - without knowing precisely which court it needs to call on.
