im pretty sure its a random sub stat (like swiftness, crit ect), then after that it depends on the item. but it can range from a random effect to like main stat (like strength).
it will not give engraving, just stats, you will can try to change the stats or bonus with silver, have 3 tryes, after it you will cant change stats or the bonus set, example the bonus can be 1% all back attack, have maybe 100 buff, is hard to get one good and you can lock it if you want, bracelets relic will give to you max 100 stats and ancient gear will give to you max 120 stats if you have luck, again is random you can check it from KR server on YT