It's a term that usually refers to certain finished or obtained chemical products that are very safe to use. The term includes MDxx (Ecstacy, MDA, MDMA, MDEA, MDPR, MDPH, MDB, MDHEA, ect) and it also includes 2C-x (2C-I, 2C-D, 2C-C, 2C-F, 2C-T-4, 2C-TFM, ect).
It's also known that sub-hallucinogenic doses of 2C-x have
interesting effects.
For example, 2C-I at low doses act as a stimulant. It's also known that mixing MDxx with a sub-hallucinogenic dose of 2C-I (For example, normal dose of MDEA + 2 to 5mg 2C-I) is so enjoyable, it's almost like MDMA with more energy and less
embarassing over-touching.