{tikz-pic} is about questions concerning “pics” in a Ti*k*Z context. A “pic” is a “short picture” that can be inserted anywhere in TikZ picture where you could also insert a node.
From the TikZ/PGF documentation, section 18 Pics: Small Pictures on Paths
A
picis ashort picture(hence the short name ...) that can be inserted anywhere in TikZ picture where you could also insert a node. Similarly to nodes, pics have a “shape” (called type to avoid confusion) that someone has defined. Each time a pic of a specified type is used, the type’s code is executed, resulting in some drawings to be added to the current picture. The syntax for adding nodes and adding pics to a picture are also very similar. The core difference is that pics are typically more complex than nodes and may consist of a whole bunch of nodes themselves together with complex paths joining them.
