This is all from memory, so I could be mistaken about some details or even the correct answer.
There used to be several differences between how MacOS and Windows handled mail attachements. There also used to be differences in how different file types were identified. Both caused problems when sending email back and forth between platforms. This has been mostly resolved over the years and incompatible email clients and file types are very rare. The era of isolated computer platforms is long gone.
These days saying that an attachment is Windows compatible probably means making sure it has an extension that Windows recognizes. In the past extensions in MacOS were not important. File type was also stored in the header, and Macs would recognize and open files without an extension.
Making an attachment Windows compatible probably mostly meant making sure it had a compatible and correct extension. It's been a long time since I've seen a Mac file without an extension (provided you've set up the Finder to display them). iOS has always honored extensions. Both of these have probably been true since Mac OSX, is based on Unix. Like Windows, Unix expects file extensions.
The Windows compatible option is probably just a legacy feature. Though I could easily be wrong.
The incompatibly of file types is also, mostly, a thing of the past. If you find you have a attachment you can't open it probably means you lack the right software. There are only a very few file types that have compatible software on only one platform.