KLayout provides two concepts for handling device variety:
Device classes are device categories. There are general categories such as resistors or MOS transistors. Specific categories can be created to represent specific incarnations - e.g. NMOS and PMOS devices. Device classes also determine how devices combine.
Device classes are documented here: LVS Device Classes.
Device extractors are the actual worker objects that analyze layout and produce devices. As for device classes, there are general device extractors. Each device extractor produces devices from a specific class.
Device extractors are documented here: LVS Device Extractors.