T-SQL what is it doing ?

I have this bit of sql code. Can anyone tell me what the "VALUE_DATE_OFFSET_NO,-20" and "VALUE_DATE_OFFSET_NO,20" gives me?
No comments in the code. What is this trting to acheive?
Is it saying to subtract 20 from the column and add 20 to the column for the declared variables?

DECLARE @MinCapacityOffset INT
DECLARE @MaxCapacityOffset INT


-- Look up the minimum and maximum offsets specified in the Capacity standards table.
SELECT            @MinCapacityOffset = MIN(ISNULL(TCAPACITY.VALUE_DATE_OFFSET_NO,-20)),
            @MaxCapacityOffset = MAX(ISNULL(TCAPACITY.VALUE_DATE_OFFSET_NO,20))
FROM            TCAPACITY