These ominous sigils are found in three different forms. They are seen on the Reincarnate, the Blessed, and the Branded. The Hollow are incapable of attaining a Soul Mark.
The Blessed: Those who are born of a parent that is marked, inherit their mark and it's properties. The Soul Mark is considered a dominant trait, and is useally passed onto the child if only one parent is marked.
The Branded: All individuals who are marked are initually branded by a member of the higher order. The Brand itself is the infant or base stage of the soul mark. Its properties are useally determined by the bestower's will, most commonly are the stipulation of property.
The Reincarnate: Those who are reborn, retain their soul mark from their previous life. This does not nessesarily mean that they remember anything of their previous, but they useally retain the same personality, quirks, and sometimes look similar in appearence to their previous life. Being reincarnated does not nessesirarily mean that your are a Reincarnate. The universe can spit you out again even if you have not been marked, however the probability is considerably unlikely.
By no means is a soul mark stagnent. The mark can be altered by those of a higher caste, or can change over time if a person signifigently changes. However, in order for a person to change their mark, they must be more powerfull that the person who bestowed the mark.
The Soul Mark is a direct representation of caste and status within the universal higharchry of the multiverse. It is universally recognized by every sanctioned government.
Soul Marks often appear on reincarnates after enduring great stress, puberty, or unfiltered planer exposure. These same qualifiers also apply to those who have inherited their marks through birthright. On occasion the soul mark appears at birth, however that is exceedingly rare.