Is The Difference Between Accounts Receivable And Its Contra Assets Called Net Realization ?

Yes there is always a different between any account and it's contra account. As you have given the example of an asset account which is Account receivable therefore, I will explain it. In fact Account receivable is an asset account, which contains the value of sales on credit. On the other hand, if a company wants to put a negative asset entry in the accounts receivable which are currently not receivables then its contra asset account can be made. The difference between the two is that, asset account increases the asset amount while contra asset account deceases it's value. Secondly, accounts receivable are always positive while it's contra asset account will be negative.