The WFCM defines a rafter tie as a structural framing member located in the lower third of the attic space that ties rafters together to resist thrust from gravity loads on the roof. The WFCM defines a collar tie as a structural member located in the upper third of the attic space that ties rafters together to resist separation of the tops of the rafters due to uplift in a ridge board configuration. Ceiling joists or rafter ties resist outward thrust of the rafters in the lower third of the attic space. See WFCM Figures 3.10b-c. See WFCM 220.127.116.11 for ridge connection requirements.
If there are no rafter ties or ceiling joists (or if the rafter ties or ceiling joists aren't adequately attached to the rafters), the roof framing must be constructed using a ridge beam so the rafters do not impose an outward thrust at the top of the wall.