1. Using the EasyGuide button is used to report an adjustment to your basis in nondeductible traditional contributions. Adjustments to you basis in nondeductible traditional IRA contribution typically come from rolling over after-tax money to one of your traditional IRAs, receiving some of an ex-spouse's basis via a transfer incident to divorce, or inheriting a spouse's IRA that you choose to treat as your own and the deceased spouse had basis in nondeductible traditional IRA contributions.
2. The EasyGuide button would normally have nothing to do with a mega-backdoor Roth because the rollover of the after-tax funds from the employer plan should go directly to the Roth account and not to a traditional IRA. However, the the after-tax funds were instead rolled over to a traditional IRA and were later converted to Roth from the traditional IRA, a basis adjustment would be needed as described in #1 above.
TurboTax's tracking of your basis in Roth IRA contributions and conversions is separate from the tracking of your basis in nondeductible traditional IRA contributions.
