When the letter S is not shown, it means that the outer packaging is a single packaging used to directly contain liquids. If the letter S is shown, it means that the packaging can be used as a single packaging to directly contain solids, or used as combination packaging to contain inner packagings containing either liquids or solids. In other words, combination packagings must always display the letter S.
Your package UN 1A2/X 1.4/120/12/J-XXXX means, it is a steel drum with removable head, can accommodate up to PG I, relative density up to 1.4, the drum sustained a hydraulic pressure test of 129 kPa, manufactured by XXXX of Japan. It is useable only to directly contain liquids. Refer DGR 18.104.22.168 (d) and (e).