DAID, or Data Attribute IDentity, values are an easy way to associate individual data points with a unique value. They allow you to associate a point within a CID with a unique identifier.
A DAID must be added to any and all points in a file that are to be actively used in the operation of the device.
What this means for a Server:
All data points that outwardly represent the operation of the user application/hardware, and;
Any points that, when changed remotely, alter the operation of the user application/hardware
What this means for a Client:
All data points in the CID file that correspond to data that the user application wants to be notified of from the connected servers.
A server might have a report that includes a status value (stVal). To change the value of the stVal and have the report publish, it would need to have a DAID set on the DAI (Data Attribute Instance - see IEC61850 definition) that has been included in the report.
A client may then subscribe to this report. To be informed of the value change being received, it would need to have the model for the server IED in it's CID file and a DAID set on the stVal.
The DAI does not need to be the same for server and client, but it must be unique in each CID file.