Inside the IAM policy, we are using the S3 bucket data source. We take the arn from the S3 bucket so that we can use it in our IAM policy. You will notice that to get the value, we use the interpolation syntax ${data.aws_s3_bucket.bucket.arn}. The opening ${ and closing } are necessary because we are inside a multi-line string. They tell terraform that we want it to evaluate this value and not use it as a string literal. The format of a data source expression is data.... You can get a full list of the attributes that a data resource provides from the documentation website of the provider.