Indemnity basis
Indemnity basis is a term of art in English civil procedure that, like so many other terms of that art (without prejudice, for example) has seeped illegitimately into the fabric of English contract law through the husbandry of over-zealous lawyers.
Derermination of costs on the "indemnity basis" means any doubt or disagreement as to the reasonableness of costs is the be awarded in favour of the recipient of the costs award. Strictly speaking this is litigation chat, but don’t be surprised to see trustees and custodians jamming it in to their security deeds.
Where the amount of costs is to be assessed on the indemnity basis, the court will resolve any doubt which it may have as to whether costs were reasonably incurred or were reasonable in amount in favour of the receiving party.
Resist.
There is an alternative: the standard basis of cost determination. If you really must let incongruent civil procedure chat infuse your contracts, prefer that.