Sealed simply means there's no dipstick and it has a more complicated and pointless method for checking the fluid level. They still have a drain plug and you can still refill the transmission.

Toyota says the fluid is "lifetime" but under heavy use or towing, you replace it after 60k and then every 30k (Which is why I don't believe "lifetime" and just recc. 100k for normal driving) a simply drain and fill is good enough for this transmissions, Toyota doesn't recommend machine flushing our sealed transmissions.

You wouldn't believe how many people we get with (usually Tundras for some reason...) that drained the Trans fluid only to find out they couldn't fill it up through the dipstick!