This is not accurate. 401(a) money is NOT a separate account. It's a particular "bucket" of money within your 401(k). its not a "separate account" that you need to go set up. Fidelity refers to these buckets as "sources" within your 401(k).

Also, there are a few different types of "in plan conversions": Converting employee pre-tax money, company pre-tax money and employee post-tax money. Converting any pre-tax money to Roth within the 401(k) is taxable event, so be careful. When converting post tax money (401(a) bucket) to Roth either within the 401(k) or rolling over to a Roth IRA (AKA Mega-Backdoor Roth), any gains on the funds are taxable. For this reason, I direct all my employee money to 401(a) and into the Stable Value Fund. With that fund there are virtually no gains, so no tax when I roll it over to Roth IRA. After the rollover, move the money to aggressive funds for non-taxable gains.