it's the only tracking number which is traceable on pos malaysia website.
7-10 days to reach malaysia.

i think priority mail is something like pos express which you buy the envelope or mail boxes from the post office with up to a certain amount of insurance.

priority is different from registered mail. USPS do have registered mail like what pos malaysia have. though registered mail is available as an option for international delivery but the tracking number is useful within US only cos you can't trace that tracking number at pos malaysia website.

from my experience, pos malaysia seemed reliable so far, unless some postman playing hanky-panky. i almost lost one parcel sent within US states using media mail which normally reached within 7-10 days but it took me more than a month to deliver from new jersey to california
but overall the USPS staffs are very helpful and efficient

most of the time, i just opt for normal post and it usually takes 2-3 weeks to reach our shore.