As an engineering student at U of T you must take 4 half year (or 2 full year) Complementary Studies (CS) Electives. At least 2 of these CS electives must be Humanities/Social Sciences (HSS) Electives. In MSE, these courses are generally taken after 1st year (unless you took a course in the summer before your first fall term, talk to your first year advisor for more about this). You as the student are responsible for making sure you take the appropriate amount of approved courses.
Find more detailed information about your electives in general here.
Some tips for students picking their courses!
You can filter for courses in the timetable builder under "Breadth Requirements / Elective Categories"
Make sure to also select the "Faculty of Ars and Sciences" when looking for HSS electives
Ask your upper years about what electives they took and what they're interested if you're having trouble picking!
Check out this google form for electives some upper years have taken and written in about (If you're an upper year please fill out this form to help your underclassmen and fellow MSEs with course selection)
In addition to the CS/HSS courses discussed above, MSE students must take a total of 5 Technical Electives (TE) which are categorized into four areas:
Biomaterials
Sustainable Materials Processing
Manufacturing with Materials
Design of Materials
At least 2 of the 5 Technical Electives must be 400/500 level. The tables in the MSE academic calendar list the Technical Electives pre-approved, but other courses can also be considered if the content is sufficiently on theme to one of the four areas or MSE as a whole. You can consult with your undergraduate advisor (Undergraduate Liaison Officer) and the Associate Chair, Undergraduate to see if a course you are considering is appropriate.