[_1] syntax

koorosh 

To explain this, I have modified the measure 

Measure 3 =
VAR _cnt =
    CALCULATE ( DISTINCTCOUNT ( Sheet1[Course  Name] ), ALLSELECTED ( Sheet1 ) )
VAR _summarizedTable =
    SUMMARIZE (
        Sheet1,
        Sheet1[Partner],
        "_1", DISTINCTCOUNT ( Sheet1[Course  Name] )
    )
RETURN
    SUMX ( FILTER ( _summarizedTable, [_1] = _cnt ), [_1] )

"_1" is a column name that you have defined in the SUMMARIZE function. 

Below is the syntax of SUMMARIZE

SUMMARIZE (<table>, <groupBy_columnName>[, <groupBy_columnName>]…[, <name>, <expression>]…)

The first parameter is Table, then group by column, then a new column name. In your case, you have specified "_1". Then expression. The output of the expression will be stored in the newly created column "_1".

Below is the syntax of SUMX.

SUMX(<table>, <expression>)

The first parameter is a table. and the second parameter is an expression. (The expression to be evaluated for each row of the table)

In your example, You wanted to pass a filtered table. That's why you have used FILTER()

Below is the syntax of FILTER()

FILTER(<table>,<filter>)

The first parameter is a table. In your original measure, you have directly specified the SUMMARIZE() that will return a table.

To explain this I have stored the result into a variable. Then the second parameter is a filter condition. ie [_1] = _cnt  

Now coming to the context of SUMX(). SUMX will return the sum of the "_1" column in the temporary table "_summarizedTable".