What is a IATA aircraft sub-type? I am currently reading docs regarding slots, where the following is mentioned:

All SCR messages in linked (turn-around format) using aircraft
subtypes only (e.g. 73G or 738 instead of 737).

Not sure what is meant by the above sentence. My understanding is that whenever you submit slot requests with SCR messages you need to use the IATA aircraft code as defined here.
The Worldwide slot guidelines also mentions:

Operating a flight in a significantly different way to the allocated
slot – including a different service type, aircraft subtype, aircraft
capacity, or origin/destination – without the prior confirmation of
the coordinator as set out in 8.10;

So what is exactly the difference between an aircraft type and an aircraft subtype? Does the link provided above refer to aircraft subtypes?
When the 737 is a type, the 737-700 (IATA: 73G) and 737-800 (IATA: 738) are sub-types. They have different capacities and differing weights. Other documents will use the word &quot;variants&quot; where your document uses &quot;subtypes&quot;.
Related info here.