Well. The definition of ISNUMERIC function is Determines whether an expression is a valid numeric type. So any expression that can be converted to numeric type is valid for ISNUMERIC to return the result as 1.
SELECT ISNUMERIC('12D2')
The result is 1. Now we can validate this using the following expression
SELECT CAST('12D2' AS FLOAT)
The result is 1200. So here the number is expressed in scientific notation and D denotes as decimal and is evaluated as 12*100 Consider another example
SELECT ISNUMERIC(',')
The result is 1. It is because comma can be converted to MONEY datatype which can accept formatted numbers.
SELECT CAST(',' AS MONEY)
The result is 0.00 So next time when you see ISNUMERIC returns 1 for an expression, it means that expression is valid for being converted to any of the numeric datatypes listedhere.