A file created on a mac can be copied over to and opened on a windows system and vice versa. A file can be hosted from FIleMaker Pro or Server and both Mac and windows clients can open the same file.
There are system differences that have to be allowed for in the design. Windows are managed differently--on windows systems, all FileMaker windows open with in a FileMaker application window but not on macs.
On either system, if the database layout specifies a font that is not installed on the computer where the file has been opened or the client that is accessing the hosted database, a different font will be substituted so care must be taken in what fonts are used.
The same text will require a bit more horizontal and vertical space on a Windows system than a Mac so layouts created on A Mac should be tested and adjusted in windows. (Such adjustment is usually not needed for Macs if the layout was created in Windows.)
And since file paths in a windows system differ from a mac system, some aspects of scripts, container fields, web viewers may not work on both platforms unless an additional effort is made to build in adjustments that detect the platform (OS) and then compute the correct file path for that OS.