Modular is a broad category which includes software that can function as both instruments and effects. This includes higher-level systems with pre-built modules that function like Eurorack, all the way down to low level DSP systems. The only requirement for them to be here is that they work as a plugin or standalone app, and can function as an instrument, effect, or MIDI processor.
This category does not include "self-contained" instruments and effects that happen to use a modular patching methodology, such as u-He Bazille or the various virtual MS-20s.