Backup Exec Cataloging question

Backup Exec Version 9 (I believe) on a Netware 6.5 server - - -

I know that open user files will not be backed up.  But my question is this:

At what point will backup exec mark an open file as unavailable for backup? Does it build an index of all files, open or not, and just pass over them if it cannot access them at the point it tries to write that open file to tape - - - or does it exclude it initally and pass over it regardless.

I have some crucila files that are practically open 24 hours a day.  They can be sut down for a bit, but not for an entire 3 hour period.  If I need to, I will create a seperate backup routine for this these crucial files that will take just a few minutes (During which time users will be able to exit, since it's a much shorter duration.) But before I go and do that, I just want to understand it better,  It's not so crucial that one of these files gets missed a few times a week . . . .but I need to make sure there are a couple good tapes, anyway.

Hope all this makes sense. Thanks