The ring is simply screwed onto the setting tool and prevents the setting tool from screwing too deep into the fully threaded insert.
Be aware you will need more rotations with the install driver on full thread inserts. For example the 51459 Triton style half threaded insert only requires 10 full turns. But if you are using a full thread 3/4 reach washer seat insert e.g. 41407 insert you would need approximately 20 full turns because this insert has twice the threads. This would NOT be noted on the 5553 instructions.  Customers that have the Ford 5553 kit that want to install fully threaded inserts need the ring for the setting tool.  EXTRA BIG-SERT M14x1.25 SPARK PLUG INSERTS CLICK HERE:
NOTE: DO NOT USE THE FORD COUNTERBORE P/n 55518 ON "NONE FORD" VEHICLES
It is not necessary to use the counterbore p/n 55518 in kit with other vehicles e.g. Toyota, Honda, BMW ect. The p/n 55518 counterbore was only added to remove a step of aluminum found only in Ford Romeo style Heads.