I've written assemblers for about 30 different ISAs (ISAE?)---most of them custom. An ISA (almost always) forms a DAG, where a node is a legal string of {01X} that is producible from the ISA; the 'X' meaning "0 or 1". An encoding is a daughter of another encoding if it has a 0 or 1 whenever the parent has a 0 or 1.

A "projection" is the imposition of a tree-like structure on the DAG, followed by a cut of the tree to make it practicalable in size. Aliasing mnemonics in the tree can cause confusion for the user---or aid in understanding.

These articles are noting that there are many ways to project the x86 ISA.