There are three levels to consider:

1. Readability for modification

2. Readability for the "what"

3. Readability for the "why"

All human-readable description in code is there to make the difference between having a piece of documentation pointing you in the right direction, and having to reverse engineer your understanding. It's an optimization problem with definite tradeoffs. Although CS professors and many tutorials will tend to encourage you towards heavy description, over-description creates space for inconsistent, misleading documentation, which is worse than "not knowing what it does."

When you see code that is dense and full of short variables, it's written favorably towards modification. It is relying on a summary comment for "what" it does, and perhaps on namespaces, scope blocks, or equivalent guards to keep names from colliding. Such code lets you reach flow state quickly once you've grokked it and are ready to begin work, because more of it fits onscreen, and you're assured the smallest amount of typing and thus can quickly try and revise ideas. The summary often gets to stay the same even if your implementation is completely different. And if lines of code are small, you enjoy a high comments-to-code ratio, at least visually.

Code that builds in the "what" through more descriptive variable names pays a price through being a little harder to actually work in, with big payoffs coming mostly where the idea cannot be captured and summarized so easily through a comment.

In your example, one might instead rework the whole layout of the code so:

var urq; /* user request type */

... (we build list "l")

/* adjustments for user request */
{
if (urq=="rns") { /* case sensitive sort by reverse alphabetical order */ ... }
}

If you aren't reusing the algorithm, inline and summarize. And if you're writing comments about "what the upper layer expects", then you(or your team) spread and sliced up the meaning of the code and created that problem; that kind of comment isn't answering a "why," it's excusing something obtuse and unintuitive in the design, and is a code smell - the premise for that comment is hiding something far worse. If the sequence of events is intentionally meaningful and there's no danger of cut-and-paste modifications getting out of sync, it doesn't need to be split into tiny pieces. Big functions can be well-organized and easy to navigate just with scope blocks, comments, and a code-folding editor.

"Why" is a complicated thing. It's not really explainable through any one comment, unless the comment is an excuse like the example you gave. The whole program has to be written towards the purpose of explaining itself(e.g. "literate programming"), yet most code is grown organically in a way that even the original programmers can only partially explain, starting with a simple problem and simple data and then gradually expanding in complexity. Experience(and unfamiliar new problem spaces) eventually turns most programmers towards the organic model, even if they're aware of top-down architecture strategies. Ultimately, a "why" has to ask Big Questions about the overall purpose of the software; software is a form of automation, and the rationale for automating things has to be continuously interrogated.