https://www.levenger.com/collections/notecards/products/perfect-timing-3-x-5-cards-90-cards?variant=42485738045589#

Slightly different than today/next/someday, Levenger sells 90 today/tomorrow/someday 3 x 5" index cards for $14.50 (or $10.00 on sale).

The Analog system has a thin metal divider that separates two sections in the base. The broad suggestion is to use this space as storage and the divider for separating the blank stock of cards from used cards. One could also separate the next/someday cards from the today cards (new and finished).

The divider serves as a tabbed divider in many systems, but here there are only a total of three slots for differentiation: one slot for one card with today's list, and two other spaces for other cards that the user can determine their uses for. This definitely makes the system incredibly simple and minimal.

It's only slightly different from the old common Park Sherman Co. desk note pad systems which sometimes had a universal calendar and one tray space for notes.

Though the Analog system clearly delineates cards of to do lists as "Today", "Next", "Someday", I've definitely seen this sort of delineation well before this system.

What other systems explicitly use this framing or similar framings?

While the today / next / someday cards are labeled, they're also color coded (white, light tan, tan) to help distinguish them.

Though not defined in the Analog system, these differently colored cards could also be used to indicate different sorts of data, though one would need to potentially ignore the pre-printed labels of "Today", "Next", "Someday".

The Analog system utilizes a simplified version of an Eisenhower matrix which we'll call "today / next / someday" as a means of prioritizing to do list items on a temporal basis.

The someday card is described as being not only for individual to do items, but "big picture" goals.