meaning - Word Hunt: A Collection of Lessons, but a subset of a Course?

I come from Stack Overflow, a site about programming, and I feel my
question is too broad for them, as it’s more about English, and less about
programming. This is more of a word-hunt than programming.

In short, I have a Database for a Web Application with different concepts
within them, like Users, Courses, Lessons, etc. These concepts can
have any name, but because I’m picky, I like to use one-word names for my
concepts.

To explain how these concepts are related: I have Lessons, which are
day-to-day exercises, like “Read this article”, or “Watch this video”.
They’re typically about learning a small subset of a broader subject, like
learning a Formula in a Math Class.

Then I have Series, which is the word in question that I would like to
replace. Anyways, a Series is simply a collection of Lessons, to cover
a general subset of a broader subject. This would be like learning about
Quadratic Equations in Algebra. Each Formula is a Lesson, Quadratic
Equations is the Series, and Algebra is the Course.

As you might have guessed, the third concept involved is Courses. These
are much broader subjects, which are broken down into Series, which are
then broken further down into Lessons.

The problem is: due to certain programming standards, Databases like to use
words where a Table (or Concept) Name is the Plural Form of the
Concept, like Courses, and each entry in the Table can then be labelled
by its Singular Form, which would be Course, in this example. However,
once we start using words like Series, where its Singular and Plural
Forms are the same, distinguishing between the two can be troublesome.

Before you post ideas, there’s a few words that I’ve already rejected,
because they weren’t quite what I was looking for:

• Group (I’m already using this for a Concept that depicts a Group of People)


• Collection (This is too vague. Something like Series has a unifying feel about it. The Lessons within a Series are related, and as such, the word should carry this meaning.)


• Theme (This means something else to programmers. A Theme is the look-and-feel of a website, like “I want my main colors to be Red and White, with a Nav-Bar on the Left”.)


• Subject (Better than theme, but still feels to specific.)


• Topic (Feels too specific, and doesn’t share the same vibe as Course and Lesson.


• Section (A “Course Section” is already a thing, if you’ve ever taken a college course. It’s basically the same Course with different Students.)


• Series (The current name, but has the issue of being a defective noun.)

To define what I’m looking for, in case it helps:

A Collection of Lessons that share some common Theme, Goal, Subject, or Topic, but still a subset of a Course, which contains many of these.

If for some reason you think that I was wrong to reject one of the words I’ve already considered, then please let me know why you think that word is really the best choice.