For IRAs inherited from a decedent who had not yet reached their RBD, the IRS allows an EDB to opt into the 10-year rule to eliminate the possibility that had they been a non-EDB under they would be in a better position under the 10-year rule in some circumstances.
Because a Roth IRA has no RBD, the participant in a Roth IRA is deemed to always die before their RBD, giving EBDs the option to elect the 10-year rule by the end of the 9th year. There is no formal election process for applying the 10-year rule under these circumstances, the EBD just needs to empty the account by the end of the 10th year if they've made this election. An EDB who is an EBD due to being the surviving spouse can assume ownership prior to the 10th year and avoid RMDs altogether while still having access to earnings in the Roth IRA without penalty (and without tax if the 5-year qualification period has been met) in earlier years.
Note that if the spouse beneficiary fails to complete an annual beneficiary RMD under life-expectancy distributions, they would normally be deemed to be the owner of the IRA. However, with the option to elect the 10-year rule, "failure" to complete an annual beneficiary RMD from the Roth IRA could just mean that they elected the 10-year rule with no annual RMDs required prior to year 10 when the entire remaining balance would become an RMD.
