Calling an individual 18650 Battery Cell China, is completely acceptable for many people. It a little cylinder thing which gives us electricity - it's easy to communicate.
However, nomenclature for engineers differs. Concerns for efficacy dictate that an adherence to standards and depending on their work doctrine, some engineers can take this very personally.
This is because normally a battery is a self-contained system capable of powering a device. The key point is battery safety. An 18650 Battery Pack on the other hand, needs additional regulation circuitry to operate safely since lithium is so chemically reactive.
The accession of a BMS is also essential to keeping a li-ions anticipated long term lifestyle. Individual cells do not have a BMS, that is the duty of the pack.
Recall the three tier system employed in building Li-ion Rechargeable Battery Cell - modules, and battery. Each category has a different set of principles in order that they can't share their titles. So in some cases the distinction between mobile and battery is essential.