I suppose the A/B designations might just be a Waterloo thing, but it's probably pretty clear that each year of schooling is split into an A and B term. When these occur will be affected by the placement of co-op terms.
Those X* classifications indicate part-time terms. At the end of my coop term that followed 2B, I was offered a full-time position for a year at the company I was working for. It was a good opportunity and so I accepted. Since the company was in Kitchener-Waterloo, I was able to take a course per term (and it was extremely convenient during the months when I worked out of one of the offices adjacent to campus). X4 was during a normal coop term (back at that same company).
If you're wondering about my course selection, here's a bit of background. My original major was Applied Math with Engineering Electives, hence the physics and electrical engineering courses in first year. After further evaluation, the available packages of engineering electives just didn't excite me all that much, so I went a different direction. While my AM courses kept me occupied in second year I also enjoyed the required STATS classes (unlike most others, apparently). I took a few more in 3A to confirm my interest and went on to complete a double major in AM and Stats.
My focus on modelling in AM (differential equations, dynamical systems, control theory), various areas of stats, and general computer technologies and programming all come together for my current studies in the Statistics-Biostatistics MMath program at Waterloo. Such school terms are designated by the inspired choice of G*.