'Substantial performance' is less than complete performance. It is a type of partial performance, and therefore gives rise to a breach of contract. However, substantial performance is so close to complete performance that it substantially gives the non-defaulting party the intended benefit of the contract.
rlf In such cases, even if the breach is a breach of condition, the non-defaulting party must treat the breach as a breach of warranty. This means that the non-defaulting party is only entitled to claim damages: performance cannot be rejected or stopped.