The policy file is used to allow a user in a Trusted AWS account to be able to assume a role in a Trusting AWS account.

assume-role-policy-trusted.json

{
    "Version": "2012-10-17",
    "Statement": [
        {
            "Action": [
                "sts:AssumeRole"
            ],
            "Effect": "Allow",
            "Resource": "arn:aws:iam::$(TrustingAccountID):role/Assume-Role-1"
        }
    ]
}