What is the purpose of AssumeRolePolicyDocument in IAM?

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"]
        }
    ]
}

The purpose of the AssumeRolePolicyDocument is to contain the trust relationship policy that grants an entity permission to assume the role.

In your example it's granting the Lambda service the ability to assume

References

https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/aws-resource-iam-role.html

https://docs.aws.amazon.com/cli/latest/reference/iam/create-role.html

StackExchange DevOps Q#5094, answer score: 16