....And to me. Shame - I read this thread hoping you'd know. ;-)
I imagine it might sometimes be used as an abbreviation for 'open to interpretation'... (as when someone politely says an idea is 'interesting' rather than 'totally impractical', or when a civil servant tries to dissuade a minister from a course of action by describing it as 'brave' or 'innovative' rather than 'foolhardy' or 'doomed to failure'). But it's not an idiomatic usage I've met - although 'open' itself is that sort of adjective: for example an 'open carriage' is not at all like an 'open sandwich' or an 'open question'. (So that you might meet something like:
"When writing a CV or resumé, avoid 'open' adjectives: a potential employer wants to know precisely what you mean and what you've done; say, for example, 'over 10,000' rather than 'innumerable'".
b