Diameter
You loiter around a tombstone to measure its diameter. You’re doing this to each tombstone, and if you loiter around long enough, you’ll soon know the diameter of every tombstone.
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You need to know the diameter of each tombstone so you can make cakes (けい) for each one. But the cakes can't be wider than the tombstones, or they'll flop off onto the ground.
An update is needed before you can continue but you have more than one WaniKani tab open.
Finish up what ever you are doing on the other tab(s), then close them to continue.
The update will complete automatically once the other tab(s) are closed.