I'm confused as to what the purpose of AssumeRolePolicyDocument is.
It seems like to create a role all you need is a parent name that you can attach policies to.

It seems like AssumeRolePolicyDocument is that parent name but somehow there's more to it.

What is this doing?

"AssumeRolePolicyDocument": { "Version": "2012-10-17", "Statement": [ { "Effect": "Allow", "Principal": { "Service": ["lambda.amazonaws.com"] }, "Action": ["sts:AssumeRole"] } ] }