This is your incremental borrowing rate as of the transition date from FAS 13 to ASC 842 or IAS 17 to IFRS 16. This is the beginning of the first comparative period to be reported in financial statements when the new standard is implemented. This is needed for any existing operating lease; it will be used as the discount rate for those leases when capitalized.
Note: The transition IBR has no effect on capital/finance leases, as their discount rate is set at the inception of the lease; the rate doesn't change at transition like ASC 840 to ASC 842.
You may make group changes of the transition incremental borrowing rate using File > New Standard Updates.