Information about the UEA timetable slotting system is available here.
Each hour on each day is allocated a unique 2-character code. However, there are also defined timetable slots that combine a number of individual hours together. For example, the CL slot combines C1+C2 (Tuesday 9-10 and 10-11).
The first letter of a slot name determines which of the individual hours are used -- i.e. slots beginning with 'A' only use hours whose code begins with 'A', and so on for 'B', 'C', etc. Therefore, there is no timetable overlap among slots which begin with different letters.
However, if you plan to take in the same semester modules whose slots begin with the same letter, you must check carefully whether their timetables clash. Evision, also available via the Portal, provides detailed timetables for each module at UEA. The tables on pages 2-4 of the document linked above also list the exact times of each defined timetable slot. This information will allow you to determine whether modules whoses slots begin with the same letter clash or not.
This information can be used to ensure that you do not select modules which clash. Once you have selected your modules, you will be issued with a precise timetable for all your classes at the beginning of each semester, including room numbers.