The exact wording varies from burning application to burning application. If you look at the image linked below this is taken from my 'quick and easy' burner. Clicking on the 'Options' button brings up the gray 'Options' box and you will see that the second line, which is ticked, reads 'Finalise disc (no further writing possible). If you untick this box then the disc will not be finalised (or closed) and you will be able to write more data to it at a later date.
If a disc of any sort is finalised or closed then you cannot write any more data to it without, for a RW disc, re-writing. But as far as I know, if you do not finalise or close the disc then you can write more data to it for all types of disc.
If a disc is still open it makes no difference what type of data you are writing. You could start with a bunch of Word documents, then add a handful of video clips and finish off on a third occasion with a set of accounts.
I hope this answers your questions. Since I use DVDs primarily for backing up and shipping photos around - I ship a lot of photos around - I have to admit that I have never actually done this, but I know the facility exists and the image is the proof ! Normally I am trying to get as close as possible to the 4.7 GB limit as I can without going over it.