Hi, In attachment you can find explainded system functions for Desigo.

First part is related to BBMD.

Regarding ABT Site (Suite used for programmign PXC 4/5/7 controllers) you can set up BBMD in Building structure tab after you click on the Controller and expanding properties for given controller.

In Desigo CC setting is done in the engineering for BACnet driver.

Hope you can find some usefull info in the attached PDF.