You are bound mostly when you distribute GPL covered works or derivative works. The licence is a-priori unconcerned whether such derivative work involve IPC or

You are bound mostly when you distribute GPL covered works or derivative works. The licence is a-priori unconcerned whether such derivative work involve IPC or dynamic linking or anything else. There is no trivial relationship between the technical kind of SW interaction and if it constitutes a derivative work or not. Sometimes projects clarify their interpretation, e.g. Linux telling (with much reason!) that merely using standard syscalls does not per-se constitute a derivative work - (and note that this is a quite stronger form of interaction than most IPC, however this does not intrinsically prevents some IPC usages to be considered as derivative, maybe especially if such IPC are added after the fact in a vain attempt to circumvent the GPL with a meaningless technical trick)