So I'm constructing a cf stack for a role in AWS and I don't know how to go about the AssumeRolePolicyDocument field when designing a role that is not resource-based. 
All the examples I've tried to look up each have a specific AWS resource designated under the "Principal" field (e.g. "Service": "ec2.amazonaws.com").
What's the correct way to go about the AssumeRolePolicyDocument field for roles that are designed for users, not resources?