In collisions the dynamically cold disks respond the most strongly and the most quickly to the disturbance, so the relative mass of the disk component is significant as a first gauge of the magnitude of the collisional effects. If there is a substantial disk, the gas fraction in the disk is also important. I propose only two variables to describe the collision itself, one for the magnitude and one for the directionality. For the former we can use Gerber and Lamb's dimensionless strength parameter. As defined, this parameter is not an observable, but it can be brought closer to observables with some additional assumptions, which are valid for the most common cases.
Most collisions probably involve members of bound galaxy groups.
In this article a qualitative estimate of S, or just an indication of whether it is small, medium or large will suffice. In the latter class are collisions between nearly equal mass galaxies. We again adopt a modified form of Schweizer's notation, using upper case letters in the Hubble variable for the primary galaxy and comparable companions, lower case for medium strength companion interactions, and subscripts for low strength encounters. The last classification variable is for directionality or angular momentum coupling. In the remainder of this article, where appropriate, the collision categories described in each section will be given in parenthesis in the section title.