To successfully build and operate next generation of our transmission system in the West in a pragmatic manner, transparent access to the current state of information on transmission flows is necessary to inform the near- and long-term transmission expansion investments. Near-term and real-time transmission flow data is vital information; it is in the public interest to have access to it, but that access is currently lacking. Such data will be needed for transmission system operation, planning, and policy decisions by all transmission-owning or -operating stakeholders, regulators, and policymakers, transmission developers, and clean energy and ratepayer advocates. However, most of the information on actual real-time and recent historical transmission flow data is limited to the grid operators, merchant transmission owners, or utilities. Further, the publicly available data is often inconsistently reported and difficult to access. This primer is intended to provide a foundational understanding of the various dimensions to data as it pertains to transmission operations and availability.