Publication buffer, containing the publication message. This will get correctly created when the publication context has been defined using the BLE_MESH_MODEL_PUB_DEFINE macro. BLE_MESH_MODEL_PUB_DEFINE(name, update, size);.
Callback for updating the publication buffer. When set to NULL, the model is assumed not to support periodic publishing. When set to non-NULL the callback will be called periodically and is expected to update bt_mesh_model_pub
.msg with a valid publication message. If the callback returns non-zero, the publication is skipped and will resume on the next periodic publishing interval.