Over the years Windows has changed, including a move from 32 Bit to 64 Bit. This has meant the .exe files that run FL Studio have also changed as we modify FL Studio to take advantage of the new features.
If you are using Windows 64 Bit - normally you should use FL64.exe. This and FL64 (scaled).exe are the 64 bit versions of FL Studio. They have access to up to 512 GB of physical memory and 8 TB of virtual address space, depending on your Windows version. The FL64 (scaled).exe will let Windows handle high-resolution scaling as explained here in the manual. It's a fix for VST GUI scaling issues. Normally you do not need to use this.
If you are using Windows 32 Bit - you should use FL.exe. The two FL (scaled).exe and FL (compatible memory).exe are for specific situations. They have access to 4GB of memory since FL Studio 11 and 2GB before that. The FL (scaled).exe will let Windows handle high-resolution scaling. The FL (compatible memory).exe has access to 2GB of memory instead of 4GB.
FL_3GB.exe can be found in FL Studio 8. It's a 32 bit program with access to 3GB or memory. This is the first version to support use of extended memory.
FL (extended memory).exe is part of FL Studio 9 and 10. It's a 32 bit program with access to 4GB of memory.
Image-Line Support Team