Thanks, but I have AU 40, and tabulation keys are slightly different. I am sorry, I can't add a picture. Please zoom on the pic here. This is exactly my model
https://www.reddit.com/r/typewriters/comments/97812h/my_first_mechanical_typewriter_an_adler_universal/

That's a little different than the decimal tabulator on my SG-1 works: You use the set/clear keys to set the tabs anywhere you want. Then, when you push the tab button with the decimal on it, the carriage will stop exactly at any of those stops. The other tab keys are for entering a certain number of digits so that the decimals always line up. E.g. When you push the tab stop button with a 1 on it, it stops one position before where you set the tab stop. The tab button with 10 stops two beforehand. The 0.1 button stops one stop after the tab stop, etc.

My understanding is that if you were filling out a form or writing a bunch of prices, you'd go set tab stops anywhere you'd want decimals to be (so everything lines up), then when you enter a particular number, you hit the tab key corresponding to the magnitude of the number you'll type. So, to type 9632.12, you'd hit the 10,000 tab key, it'd zoom to 4 characters before the stop, and you'd type your number in, which would cause the decimal to be exactly at the stop you set. If on the next line, you entered 1.99, you'd hit the 1 tab key, enter 1.99, and the decimal would line up perfectly with the preceding line. 

So, for general prose writing, I never use anything but the decimal tab key. 

This is the only (functioning) decimal tabulator machine I've got, so I'm not sure if this is true for all brands.

Apologies for my clumsy writing. I think we're actually in agreement. My SG-1 doesn't have a "Tab" key, but has set of decimal tab keys like those in the Adler image above and from reddit.

Here's my SG-1:



They work just like the Underwood Duane Jensen demos here: https://youtu.be/NsPfVqv7tLU?si=ZIgZHXBMZBpTciu5&t=56. When I set a tab stop with the "set" key anywhere on the line, the "." tab key is the only key that goes to that spot. The other keys go some number of spaces before that spot, depending on the number of digits. These other tab buttons are not independent of each other, but are simply some number of spaces from wherever the tab stop is set.

I do disagree slightly with Duane from the video: I think you're supposed to set the tab where the decimal place is supposed to go, then you press the tab key that corresponds to the number of digits you have to type: 1 for one digit, 10 for 2 digits, 100 for 3 digits, etc. In the video, he uses the "." key for the first digit instead of the decimal. It'll still get the job done, but you have to press the tab key for one number of digits fewer than the number you're entering. E.g. "." for 1 digit, 1 for 2 digits, 10 for 3 digits, etc. It makes more sense to me to set the tab stop for where you want the decimal to be.

My comment about prose writing is that since all these tab keys merely give you slight position shifts from wherever you set the tab, then they're not useful outside of typing tables of numbers. When I'm writing text, I only use the "." key as my effective "tab" key. 
 

Which one ?
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