Lets say you had a device that had two readings the Primary reading and a second reading. Like a light detector with an intensity and color of the light.
How it`s sense to read/write characteristic ?
A verbatim from the BLE Spec: "A primary service is a service that provides the primary functionality of a device. A secondary service
is a service that provides auxiliary functionality of a device and is referenced from at least one primary service on the device. A secondary service is a service that is only intended to be referenced from a primary service or another secondary service or other higher layer specification. A secondary service is only relevant in the context of the entity that references it."
Coming to writing/reading the characteristics in these services, there are no differences between the two. You can read/write to characteristics in secondary service just as characteristics in the primary service.