You may choose to categorise grade items for organisation for simplification, or to enable the use of sub-totals when calculating the topic total.
Locate the Administration block (usually on the left-hand side)
Click Setup, Gradebook setup
Click Add category
Give the category a Name
Choose an appropriate Aggregation method. This is the method used to combine all grades within that category to create a sub-total. The two most common options are Sum of grades (adding all grades together to create a sub-total), or Simple weighted mean of grades (averaging the grades, giving equal weight to each grade item).
Click Save changes
To move grade items into the newly created category, select the checkbox next to each grade item, and use the Move selected items to menu to move the items into the category you just created.