Concept:
Patient counselling is an important responsibility of pharmacists.
Step 1: Meaning of patient counselling.
Patient counselling means explaining the proper use of medicines, dose, timing, side effects, precautions, and storage instructions.
Step 2: Pharmacy Practice Regulations.
Under Pharmacy Practice Regulations-2015, pharmacists are expected to provide professional pharmaceutical care.
Step 3: Professional duty.
Patient counselling is not optional. It is a mandatory professional responsibility.
Step 4: Final answer.
Therefore, patient counselling is a mandatory professional duty.
\[
\boxed{\text{Mandatory professional duty}}
\]