I've classified elements by how they can or can't be made by reactions:

Raw elements:

Raw elements can't be made by reactions, but can be made into other elements by reactions; examples include but are not limited to C4, NITR, GLOW, and GEL.

Exhausted elements:

Exhausted elements can be made by reactions but can't be made into anything by reactions; two prime examples are GOO and GRVT.  GLAS can be turned into BGLA and molten GLAS, but BGLA can only turn into molten GLAS and molten GLAS can only turn into GLAS.  Because all of GLAS's reactions cycle back to GLAS, GLAS creates a dead end in production chains and thus still counts as an exhausted element.

Inert elements:

Inert elements can't be made by reactions, nor can they become other elements by reactions; they're many examples of inert elements, the most notable of which is DMND.

Line elements:

Line elements are elements that can be made by reaction and can become other elements by reactions; so named because they're in the middle of prodution lines.  They're the most common type of element in tpt, with neutrons having the most reactions of any element.

Things that don't count as reactions:

CONV replacing things

CRAY or DRAY creating or replacing things

Clone variants creating things

RSST/RSSS becoming their ctypes

Virus infecting things

SOAP curing virus into original elements

Stacking non-energy elements into BHOL

If these counted as reactions, element reactions would lose a significant amount of uniqueness.