A provisional sum is an amount included in a quote for work that will probably be needed but cannot be measured or priced accurately until the job is under way. Rock under the topsoil, rotten timber behind cladding, the length of a drain that has to be dug up, the number of tree roots in the way: each is real, likely and unknowable from the surface. Instead of guessing high to protect yourself or leaving it out and arguing later, you put in an honest allowance, say how it will be measured and charged, and adjust the final bill up or down to what the work actually cost.
It differs from a prime cost item, which covers supply of a chosen fitting only. A provisional sum covers the work itself, and usually materials too. Some states require provisional sums in domestic building contracts to be reasonable estimates and to be set out in a particular way, so check with your state's building or fair trading body before relying on them for residential building work.
In practice
A concreter quotes a driveway and suspects rock in the top 200 millimetres at one end. The quote includes a provisional sum of $1,500 for rock excavation, based on ten hours of rock breaking at $150 an hour, and the note says the sum is adjusted to the hours actually used at that rate. If the rock takes six hours, the customer is credited $600. If it takes fourteen, $600 is added. In both cases the customer knew the rate before they said yes, and nobody is surprised.
Provisional sums only work if they are honest. An allowance set low to make the quote look cheaper is quickly found out, and the extra charge reads as a trick even when the work was genuinely needed. Set it at what you really expect, record the hours or quantities as you go, photograph the conditions, and show the working on the final invoice.
In Quote Desk
A provisional sum goes on as a typed-in line, described plainly with its basis, for example "Provisional sum: rock excavation, 10 hours at $150". How it will be reconciled goes in the note. The help article on building a quote covers typed-in lines.