The difference between calendered items and incubated items (presumably you're referring to date-related incubated items) is not necessarily where they're put—both may end up in the same place after you clarify them—but what they mean to you. If an item is something I'm committed to do on a specific day or at a specific time, then I would put it on my calendar in a day- or time-specific slot. If it is something I'm committed only to review on a certain day (or after a certain date), then I would put it on my calendar as well, as I don't have a physical tickler file. As long as the eventual organization of my things reflects their meaning, I don't necessarily care where they end up.
I recently made adding things to my calendar a little clearer and easier by splitting up my single calendar in my calendar software into 3 separate calendars—day-specific commitments (yellow), day-specific information (blue), and time-specific commitments (red). One could call these training wheels, but I enjoy that they force me 1) to put onto my calendar only things that really go there and 2) to refine my entries’ wording and organization so they’re easier to understand later. I recently added a "Due Projects" calendar (also red), which holds project and subproject deadlines. Very useful.
I strategically chose those colors so that I could visually decode at a glance the commitments of any given day. A lot of red all-day events indicates a lot of due projects or subprojects, for example.
By the way, I've created a tickler function for my iOS devices that I've been using since the beginning of the year. I find it critical for incubating items to be reconsidered not necessarily on specific days, but after specific starting dates.
1. First, I created a calendar called "Tickler" and gave it a color that no other calendar was using.
2. Next, I created a shortcut that took all of the all-day events on the Tickler calendar for the current day and put them in my Things 3 inbox as individual to-dos.
3. Then I created an automation that ran this shortcut every morning at 1 a.m.
4. Finally, I hid the Tickler calendar so I wouldn't see it when I looked at my calendar.
Functionally, this is a digital version of the physical tickler file. Whenever I add an all-day event to a specific date on the Tickler calendar, it will appear in my Things 3 inbox for processing starting that same date. (For items I want to reconsider only on specific days, I just do the classic "put it in day-specific information" trick.)
I see your question as an instance of how the clarifying process may proceed differently for different people, but will often result the same organization in the end.
An example of this phenomenon in my own life is the creation of waiting-for items when I’m processing. I find that if I proceed down the clarifying diagram intuitively, I never reach the delegation box; I'm not involved in any situations that would ever cause me to. Instead, I find that my waiting-for creation often goes something like this: “No, I can't do anything about this until Person X does action Y/delivers deliverable Z. This is not actionable. I’ll put a reminder of the thing I'm waiting for on a list of things I’m waiting for.” In reality, I've organized how one would expect, but the clarifying process leading me to that was quite different.
I get away with such shenanigans by keeping in mind that clarifying is not the end; it’s simply the means to the end of organization, which itself is the means to the ends of reviewing and engaging. Whenever I feel like the standard clarifying process is too ambiguous or nonfunctional, I zoom out and consider that I’m merely in the 2nd stage of the whole process—determining what my inputs mean so that I can proceed and put them where they go—and that this stage has a purpose beyond being absolutely perfectly executed in itself.
In short, if my intention is to organize my stuff, I'll naturally clarify it without much of an issue.