Mail app
On the mail app on iPad, there is an icon with 2 squares and when you hit it, it just looks like it takes a screen shot, does it do anything else. It doesn’t save the picture and doesn’t do anything I can see.
iPad Pro, iPadOS 15
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The highlighted icon (with two overlapping squares) does not trigger a screenshot. The icon that you describe triggers display of the Mail Shelf (the Shelf being a new feature of iPadOS multitasking environment). Tapping the icon summons the shelf view - which will show you a thumbnail image of all the running instances of the Mail App. Where multiple instances are running, a simple tap of a thumbnail will instantly switch to that view.
The iPadOS multitasking environment permits you to run multiple “instances” of the same App, such as Mail - with each instance, perhaps having a different open inbox.
For example, if you have three running instances of Mail, tapping the icon you describe will open the shelf with three thumbnails - each corresponding to each running instance of Mail. Simply tapping a thumbnail will switch directly to that running instance of the Mail App.
This article from Appleinsider has a useful explanation of this new feature along with many of the updates to multitasking:
The iPad User Guide is also a rich source of frequently overlooked information. The Guide is available both online using a web browser (such as Safari), or as an Apple Books download:
iPad User Guide - Apple Support
https://books.apple.com/book/id1567104892