Tags and accounts are powerful tools that Amazon provides for separating and allocating your costs. Here are some resources for understanding how AWS tags and accounts work: 

Organizing costs with tags and linked accounts

Three simple tagging rules

The nuances of tagging

Video: Mastering the fundamentals of AWS cost management

Video: How to split up your AWS bill

Video: AWS cost allocation using tags and linked accounts

Understanding the DBR

AWS cost by tag

AWS resources that can’t be tagged