NESTED IF

Anonymous , try this calculated column:

DAX Column = 
VAR Current_Index = TableName[Index]
VAR Prev_Index = Current_Index - 1
VAR Prev_OPDSC = 
	CALCULATE(
		MIN(TableName[OPDSC])
		,ALL(TableName)
		,TableName[INDEX] = Prev_Index
	)
VAR Result = 
	SWITCH( TRUE()
		,TableName[CODE] = "M" 
			&& TableName[OPDSC] = "2SH,2LG"
				, "M"
		,TableName[CODE] = "M" 
			&& TableName[OPDSC] <> "2SH,2LG"
			&& TableName[OPDSC] = Prev_OPDSC
				, "M/A/B"
		,TableName[CODE] = "M" 
			&& TableName[OPDSC] <> "2SH,2LG"
			&& TableName[OPDSC] <> Prev_OPDSC
				, "A/B"
		,TableName[CODE] <> "M"
				,TableName[CODE]
	)
RETURN
Result

This code first determines what the previous index is, and gets the [OPDSC] for that row.

The SWITCH() TRUE() pattern lets you evaluate multiple statements, and returns the first result that is true.

Just make sure to replace all instances of "TableName" with the name of your table!

Cheers,

~ Chris