.9% saline is the normal one used for most nebulizing. However, there have been some studies that show MAC bacteria cannot survive in 7% saline, so many of us with a history of MAC infections nebulize with the higher dosage in hopes of keeping bacteria colonies in our lungs under control. Dome cannot tolerate the 7%, so use 3% as a "happy medium".
Here is a link to a highly technical article for you to wade through: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4850692/pdf/13104_2016_Article_2054.pdf
Of most interest is the table on page 4.

I did notice on another discussion that you recently had the experience of coughing up blood, so you should definitely consult your dosc before trying the higher concentration, because it does tend to irritate the lungs.
Sue