I'm not completely sure, but this means the added amount of money to the person salary that will leave him after the tax deduction with his net salary. For example, if a person earn 1000 USD net, there is a need to backward calculate the amount of tax in order to leave him with the above net sum. So, if the taxes are 500 USD this person salary is actually 1500 USD as far as his employer is concerned.
I think that under this clause also included the sums for different expences as for example car and travel costs, etc.

I'm not totally sure so it is best to wait for the opinion of the more economically educated people.

meaning amounts charged against the employee's income