Most poor sealcoating jobs start before the sealer is applied. A clean, dry surface helps the sealer bond — dirt, loose gravel, oil spots, and existing flaking sealer should all be addressed first. Cracks should not be an afterthought either: a proposal should say whether cracks are included, excluded, or priced separately, and whether they're filled with cold pour material or hot rubber crack seal.
A single coat may be used for driveways or light maintenance; two coats are common for commercial lots and higher-wear areas. Don't assume two coats are included unless the proposal says so.
In hot markets you may also be offered a reflective, light-colored cool seal instead of a black one. It is applied the same way by the same crew; what changes is the material price, usually the number of coats, and the striping contrast afterward.