What is the scope of the expressions serious crime and grave misconduct occurring in Rule 8(1)?

What is the scope of the expressions serious crime and grave
misconduct occurring in Rule 8(1)?
The expressions have not been defined in the rule. However, Explanation (a) and (b)
under Rule 8 indicate that phrases include violation of the provisions of Official
Secrets Act 1923. Reference to the above is an indication of the level of the
violations which warrant action under Rule 8 of the Pensions Rules.