External activities are part of the standard IDEF0 methodology. They are higher level context activities, numbered negatively (A-11, A-12, etc) that produce and consume information to and from the A-0 Context diagram.

Usually IDEF0 models start with a single context diagram (A-0) with a single activity (A0) and decompose from there down to leaf activities. FIPS 183 IDEF0 standard provides for higher level context activities (A-1, A-2, etc) and calls for an A-1 diagram with 3 activities - A-11, A0, and A-12 that then decomposes to the single A-0 diagram with a (the same) single A0 activity. Thus, higher level context activities provide termination for lower order context activities.

The rule adopted is to provide at least two context activities using the External Activity symbol on the context diagram and nowhere else on any other diagram.

The following image shows the External Activity command in the user interface.

These external activities provide termination for forming Information Exchanges between themselves and leaf activities later on in lower levels of the activity model. One External activity produces an input to the A0 context activity and one consumes an output from the A0 activity. There can be more than two external activities, as many as necessary — one or more providing inputs to the A0 Context activity and/or one or more consuming outputs from the A0 Context activity.