Let's assume an employee making $100,000 can contribute 4% PRE-TAX to their 401a, or $4,000.
This employee then has $14,000 remaining in eligibility for PRE-TAX contributions to a 403b.
Both the 401a and 403b are TAX DEFERRED accounts - the holder will eventually need to pay taxes when the funds are spent, but that taxable-event is DEFERRED until such time as the holder withdraws funds from these accounts to pay expenses in retirement (or anytime after age 59.5).