Audit tests

Dear tutor,

In the textbook, I sometimes come across the phrase that audit procedures are designed to test against the validity of the transaction figure and the balances.

In this regard, I do not understand what validity of a transaction figure or balances refer to? Does the “validity” means the same as occurrence assertions of underlying transactions and existence assertion of a balance?

Could validity be interpreted in a wider context to cover all the assertions like completeness and valuation of the transaction tested against?

Would you mind giving me some guidance on what assertion are there to be fulfilled in testing against validity?

Thank you for your help.