Countries are more likely to diversify into new, more sophisticated goods that share "know-how" (e.g. non-tradable inputs, technology, skills, etc.) with goods that are currently being exported.
Most commonly, the tradable sector consists largely of sectors of the manufacturing industry, while the non-tradable sector consists of services, including health, education, retail and construction.
In the model, there is a non-tradable sector (which includes services) and two tradable sectors: the booming sector, and the lagging (or non-booming) tradable sector.