This thing is a customizable bookmark.
I usually print it very thin (0.6 mm) and keep a few in my notebook to mark specific pages. The thin bookmark are not very sturdy but you can use multiple ones without damaging the pages.
You can freely change the text and text size as well as the bookmark's dimensions. Here are the signification of every customizable parameter:

Size=30; //lateral dimention of the of the bookmark
Length=60; //lenght of the part of the bookmark inside the book
Text_dimention=10; // height of the text zone
Thikness=0.7; // thikness of the part
Text_Size=5.5; // Size of the text
Text="Ref. 2"; // texte to be included

Please be reminded that letters with "island" features like "a, e, r, o, p, d, b, g" will include a hole after printing.

I discovered this page after finishing this project: https://www.thingiverse.com/thing:1039106
You may want to look at it if you want more options to customize your bookmark.

For detailed explanations on how to use a customizable part look here: https://www.instructables.com/Learn-How-to-Use-Customizable-3D-Models-With-OpenS/