The description of the phenological development stages within PEP725 is based on the BBCH scale (for further information, see Wikipedia). This is a decimal system that primarily divides plant development into stages ranging from 0 (germination) to 9 (senescence); within each primary class, there is a further ten-stage subdivision. In total, the BBCH scale thus provides 100 stages of plant development.

However, some phenological observations do not quite fit into this framework, such as those used to track agricultural activities (first cut for hay harvesting), process area-wide information (25% of permanent grassland shows fresh green), or capture certain phenomena in greater detail (leaf color change in 10% increments).

For this reason, the phase_id was introduced; in the range 0–99, it corresponds to the BBCH scale—the range starting at 100 consists of custom definitions that can be expanded as needed.