Concept: In cataloguing, the main entry is the principal entry of a document. It is usually made under author, corporate body or title depending on cataloguing rules.
Step 1: Check Assertion A.
The main entry is generally made under the person, persons or corporate body responsible for the work.
\[
A = \text{Correct}
\]
Step 2: Check Reason R.
Sometimes the main entry is made under the title, especially when author responsibility is not clear or when cataloguing rules require title main entry.
\[
R = \text{Correct}
\]
Step 3: Check explanation.
Reason R gives an exception or additional rule. It does not explain why main entry is generally made under person or corporate body.
Therefore, both A and R are correct, but R is not the correct explanation of A.