Subcutaneous injection is to inject drugs into the soft tissue under the skin. (Many injections are like this, and the upper arm and the outer thigh are generally selected.)

Intradermal injection is to inject drugs below the epidermis of the skin, generally above the dermis, but not below the subcutaneous tissue. (some vaccines are like this, as well as drug sensitivity tests and pre-operation of local anesthesia. )

Intramuscular injection is deeper and reaches the muscle tissue directly. (Generally, the needles on PP are like this.)