IEC 60870-5-104 standard

My question is interrogation group ()COT 21-36) is part of the IEC 60870-5-104 standard or not?
if we look for a definition of this standard:
IEC 60870-5-101 [IEC101] is a standard for power system monitoring, control & associated communications for telecontrol, teleprotection, and associated telecommunications for electric power systems.
Features
• Possibility of classifying the data into different groups (1-16) to get the data according to the group by issuing specific group interrogation commands from the master
IEC 60870-5-104 (IEC 104) protocol is an extension of IEC 101 protocol with the changes in transport, network, link & physical layer services to suit the complete network access.
that mean that standard IEC 60870-5-104 must have interrogated by interrogation group 1 (Cause of transmission ( COT 21-36)) or not , it's not part from the IEC 60870-5-104 standard

The group interrogation - COT 21..36 is part of -5-101 inherited then by -5-104 part of the Standard. But to support it is not mandatory; only the support of general interrogation - COT 20 - is mandatory. After the connection is established the Master must send the general interrogation (COT=20) to the Slave. The Slave must close the connection when it does not get GI at start.

The group interrogation can be used to "manual" request parts of data; only when data in slave was additionally grouped. IMHO not very useful, the data should be already on master up-to-date - received by first GI and then spontaneously updated. And for additional periodical "backup" transmission there is better mechanism - background scan. We haven't implemented group interrogation.
BTW, for next zenon version the bacground scan will be smothly extended.

Thank you Ursula for your answer.
My question, it's just the client requests that we must follow the IEC 60870-5-104 standard, if we say that "scan group" is not a mandatory part of the IEC 60870-5-104 standard, and we are not forced to provide this.
My answer is correct or incorrect ?

in energy(substation), if the the client requests that we must follow the IEC 60870-5-104 standard, and the client want to have a scan group in SCADA-RTU system.:confused:
i can't find information about IEC 60870-5-104 standard with requirement that this standard must include scan group(COT 21-36).
By don't using Scan group in our system and saying that "scan group" is not a mandatory part of the IEC 60870-5-104 standard, and we are not forced to provide this scan group in our system
We respect the IEC 60870-5-104 standard or not?:confused:

The points in the Standard which are not mandatory are just optional. A system respects the Standard when it supports all mandatory points. The system can additionally support some (or all) optional points. Then to be still conform to the Standard, the system must support such optional features exactly like defined in the Standard.

The "scan group" is not mandatory, is optional. Your customer wants to get some optional feature you don't have. So you are able to deliver a system which absolutely respects the Standard, but the system will not cover all wishes of this customer.

I don't understand why the group interrogation is for this customer important. There are better mechanisms to use in systems basing on this Standard.

You can assure your customer that the whole (or any part) of data of RTU can be absolutely correctly, reliable and on-time acquired by your system. Even without group interrogation, so without additional trafic.

Thanks a lot, that really the missing information that I need. Now, We can propose a solution and be sure that we respect the IEC 60870-5-104 standard.