What does it mean?
A provisional sum is an allowance for work that is not sufficiently defined to be fully priced at the relevant stage. It is not the same as a firm quotation for an agreed scope. The contract and tender documents govern how the sum is instructed, adjusted and accounted for, including any treatment of associated attendance or programme effects.
In practice
A budget might carry an illustrative 10,000 allowance for drainage alterations before the design is complete. Once scope and pricing are agreed, the actual valuation can differ from that initial allowance.
Keep in mind
For a project record, note the document reference, revision and agreed measurement or contractual basis beside the quantity. Keep assumed information visibly separate from confirmed information. If two parties use different definitions or inclusions, resolve that difference before comparing amounts. These explanations are general estimating context; the wording of the applicable contract and its administration process must be checked for the particular job.