[sly010]

There is in fact a 3rd: build time dependencies.

Control: Object/module/function A calls object/module/function B.

Data: The call can either push or pull data.

Build dependency: The call can be direct (A depends on B) or indirect through an interface/callback/etc (both A and B depends on the interface).

Ideally every design document includes all 3 as separate diagrams.

[sly010]

Then of course there is the whole fractal nature of software. As you add more detail, the arrows can flip flop around. Polling in a lower level of a stack can very much be used as a push mechanism. (e.g. USB interrupts are in fact the host polling the device)

This is why communicating architecture is often as hard if not harder than implementing it.