Which functional groups are considered in iupac naming`s suffix when another with highest priority is present?also when is oxo and firmly specifically used for aldehyde no 16 also

Dear student,
While numbering the parent chain it should begin with high priority group and suffix should be the high priority group.
16) 3,4-Dioxo Pentanoic acid carboxylic acid is used as suffix since it has highest priority, and other two C=O groups should be called oxo if their C are not included while counting in parent chain as in this case. your number 5 should be on terminal -CH₃not C=O