I learned that I can name an APEX graph as <nodename>__REST to get rest position of other graph, or <nodename>_null_pos to get the null position.
Is there any documents for this kind of naming convention?

there is no such thig as _null_pos, that sounds like a random suffix, I see mostly these 2 suffixes in the component::AddFusionGraph that do something useful

1.
<nodename>__REST on Value<T> node
- plugged into TransformObject type node will compute its xform and sets the value of restlocal input to it regardless of which input is plugged into (I'm not sure why this is so hardcoded, meaning even Value<Int> plugged into xord would still set restlocal input to the full matrix transform based on the <nodename> )
- plugged into any other node will set the value of the actual input that it's plugged into

2.
<nodename>__MIRROR on Value<Bool> node
- will take the value of Mirror checkbox from component UI and put it into node input it is plugged into