Without having read the other parts of the contract and therefore not possessing the necessary background to reply with certainty, I would guess that the sentence means: "The whole agreement will be determined by the exact total weight and the total distance transported."
This is only a guess. You should provide more information for a more precise answer.
"The price of each unit of service shall be that shown on the approved Commercial offer, i.e Kshs 25 per metric ton,per Km including all taxes and allowances. The entire contract shall be determined by the exact total tonnages and kms covered"
The issue here is, a unit of service is carrying one ton for 1km distance =
Thank you for your reply and the additional information.
I believe that I understand the problem. However, this appears to fall under the purview of the interpretation of a legal contract. I am not an attorney and I am not qualified to offer an opinion on the meaning of words in a contract.
You may wish to engage the services of an attorney.