In conventional notation, bracketed notes can be used for a number of purposes. Often they are used to show passages which are different when a section is repeated. At first, I thought this would most likely be the case here; however, the first use of bracketed notes is for all of the first two lines, which don't appear to be repeated.
Another use for bracketed passages in conventional notation, is to show notes which are not in the main melodic part (backing vocals in a vocal part, for instance). This seems to be the most likely use here, although I don't know this piece, so can't be sure. So, I would imagine that the bracketed notes are not to be played by the main instrument, and are therefore either optional or simply "cue-notes" showing notes heard in another musical part. This would explain the fact that there are both bracketed and unbracketed notes on line 9.