GATT (Services and Characteristics) - Getting Started with Bluetooth Low Energy - page 57

Figure 4-1. GATT data hierarchy
The attributes in a GATT server are grouped into services , each of which can contain
zero or more characteristics . These characteristics, in turn, can include zero or more
descriptors . This hierarchy is strictly enforced for any device claiming GATT compati‐
bility (essentially, all BLE devices sold), which means that all attributes in a GATT server
are included in one of these three categories, with no exceptions. No dangling attributes
can live outside of this hierarchy, as exchanging data between BLE devices depends on
it.
For most types of data in the GATT hierarchy, it is important to differentiate between
their definition (the whole group of attributes that make it up) and the declaration . The
declaration is a single attribute that is always placed first (in increasing handle order)
within the definition and that introduces most of the metadata about the data that
follows. All declarations have read-only permissions with no security required, because
they cannot contain sensitive data. They are only structural attributes that allow the
client to find out and discover the layout and nature of the attributes on the server.