The user agent parser will tell you if the user agent you submit contains any fragments we've marked as "restricted". These include things like common HTML tags, some sorts of personal/business information (some company started sending the branch street address for all it's remote offices in its employees user agents, which is very bad!), as well as a list of "naughty" words.
The is_restricted key is useful to check if you are going to display your user agents anywhere sensitive. We do our best to mark it true if we think it's probably not a string which should appear to customers or anywhere public facing.
Remember, user agents are completely "free text" fields that can be changed to absolutely anything, so if you are showing them indiscriminately, you might run in to trouble. We use this same check to determine whether each user agent should be shown on our user agent listing.