I like this,
"NPT is the use of dirt (any dirt) as the primary media under a containing cap material"
"NPT - natural planted tank
Broad term for soil based aquariums"
Keep it broad, include MTS, MGOCPM, top soil, potting soil, compost and mixtures.
To me though its anything soil based not specifically produced and sold as an aquarium substrate. But I'll accept those using soil based aquarium substrates like AS as part of the Order.