As rightly said by Ashish, you can have conveyance distributed grade-wise/designation-wise. At the end, when we compute the tax liability, the tax authorities will allow an exemption of a mere Rs. 800 per month for conveyance, irrespective of the designation or the mode of transport used.
This means that if I pay, say, Rs. 1500 as conveyance to my employees, it is very much allowed. However, when I calculate his tax liability and TDS, I will be able to give a tax exemption only on Rs. 800, and the remaining Rs. 700 will be taxed.
As a result, many companies stick to the standard Rs. 800 per month as conveyance so that we can try to save tax on the additional conveyance amount given to the employee.
Hope this clears your doubt.