I have never heard of a CO2 tube made specifically for engraving and not cutting. 60W should be 60W, so either tube should both engrave and cut. I am guessing it is a miscommunication or misunderstanding on their part. Their website says that the T has an "excellent" spot size, perhaps that makes it more effective for cutting (but should also make it better for engraving I would assume).
Looking at the specs at Lightobject and Aliexpress, I would bet that Lightobject is selling the C70 for the 60W tube. Some specs.:
C60: 50mm diameter/1200mm length
C70: 55mm diameter/1250mm length
T75: 80mm diameter/1050mm length.
https://www.lightobject.com/SPT-60W-...-Tube-P70.aspx
https://www.aliexpress.com/item/Adve...789442306.html