How to convert or rewrite Access IIF Statement in SQL Server

I thought the IIf statements returns one value if the condtion is true or false BUT This IIf statement in Access returns the field and it values.

IIf([A1]![KPr],[A1]![Kat],IIf([Data2]![Kat],[Data2]![Kat],[Data1]![Kat])),

the table left join in the from clause I'm try to realize this statement in 'SQL Server' using 'CASE WHEN' but it also accepts a true or false condition.

How can I understand and realize this statement.