AWS data iam_policy requirements

Hello to all,

I was looking how to get the ARN of a IAM policy and use it in a resource iam_role_policy_attachment. So looking the documentation and I saw that the only requirement for data_iam_policy is… ARN.
So beside of having something more understandable and shorter and if you need something other than ARN as reference, what is the reason to having as attribute reference the ARN since I need to provide it?
I can easily use a variable instead of using a data source.

Is it something that I don’t get from the documentation?