DAX Commands Count

The below measure works for the following sample data:

Measure = 
CALCULATE( 
    SUM( 'Main Claim Data'[Net] ),
    'Main Claim Data'[ClassOfBusiness] = "OT",
    FILTER( 
        'Main Claim Data',
        ISNUMBER( SEARCH( "REC", 'Main Claim Data'[PolicyCode], , BLANK() ) )
    )
)

Note that if the check against "REC" needs to be case-sensitive, then you'll want to use FIND instead of SEARCH.

Sure can. See the below. I incorporated CONTAINSSTRING used by goncalogeraldes as that has the same effect as ISNUMBER/SEARCH but is more readable (+1 didn't know about those functions). Also, I refreshed myself on best practices for multiple filter arguments at the following, which is why the syntax is a little different from before for the filters: https://www.sqlbi.com/articles/specifying-multiple-filter-conditions-in-calculate/

Measure2 = 
CALCULATE( 
    SUM( 'Main Claim Data'[Net] ),
    KEEPFILTERS( 'Main Claim Data'[ClassOfBusiness] = "OT" ),
    KEEPFILTERS( CONTAINSSTRING( 'Main Claim Data'[PolicyCode], "REC" ) ), 
    KEEPFILTERS( CONTAINSSTRING( 'Main Claim Data'[Reference], "ADMIN" ) )
)

Output (Measure is from previous answer, Measure2 is the new measure above):

Sample data used:

As goncalogeraldes mentioned, use CONTAINSSTRINGEXACT instead of CONTAINSSTRING if you need case-sensitivity.

Also, I'm assuming the new filter is another AND criteria. As long as you need AND logic on criteria, you can add each as a separate filter argument in CALCULATE. If you need OR logic, you'll have to combine (just what needs to be OR'd) in one filter argument.