Coverlay serves as a solder resist for flexible printed circuit boards. Conventional soldermasks have only a limited bendability, sofor flex-circuits that require greater bendability, coverlay is glued on to protect the copper structure.
The disadvantage is that these films must normally be cut or drilled, and can not be developed like photosensitive solder resist. Thus, only larger structures in a rectangular shape are possible, as well as small aperture exemptions that are always around (drilled). For flexible circuit boards with finepitch SMD areas, coverlay is only of limited use.
At Multi-CB the coverlay is lasered, and therefore also usable for finer structures!
See also: Flex- and Rigid-Flex PCBs