Here's my official answer to your question:
Imports into FileMaker databases are found set sensitive.
If you're importing records in a method that acts on existing records (update matching), FileMaker uses the found set showing in the layout on your active window to do the matching on rather than the entire table itself.
Regarding it's localness (per your comment above), it allows you to do more precise imports. In a case where you want to make sure you only match for specific records (e.g. you have a spreadsheet of data from employees at company A and you only want to update employee records for company A) you could perform a find and limit the found set to just your desired records before importing with matching turned on. This wat the import will ONLY look at the records in your found set to do it's evaluation. This means less processing because FM has to look at fewer records and also less risk that you're going to find a match that you didn't want (depending on what your match criteria is).
I'm having a hard time finding a good and up-to-date reference for you. All I can find is this one that is form FM10 days on Format. I would suggest bookmarking the FileMaker 13 help pages. It's the same set of help documents available when you use the Help menu in FileMaker Pro, but I find it much easier to search the docs via a browser.