The tradition of patronage between patrons and their proteges has existed at least as long as the Compact of Arvum itself, and represents a time honored way of powerful members of the nobility recognizing (and sometimes taking credit for) exceptional examples from a lower social class. Officially the practice arises from the responsibility of powerful peers of the realm, visible to all society at large, to cultivate the talents of those of lower births. In practice, it is often used by canny highborn peers to garner the praise for a peer's of lower birth or an exceptional commoner's ascent, and in turn for lower peers or commoners to gain greater social notice through their association to one of the highest born.
Perhaps the most famous historical example of a patron and protege is Queen Alarice the Great and the first Radiant of the Whisper House, Caithness. While a commoner of low birth with little experience with the peerage, Caithness nonetheless possessed great wit and social grace, and Alarice recognized the enormous potential of the young woman and helped instruct her in courtly etiquette. Introduced as Queen Alarice's protege, Caithness became a force at court, often acting on behalf of the queen and the future Radiant's great charm reflected well upon the queen and won her allies, as the esteem of both women steadily rose.
In modern days, a noble typically picks no more than a handful of commoners or lower born nobles that they believe can represent their interests and increase their social standing, and for the lower born it represents a great opportunity to be seen and associated someone of high visibility in order to leave their own mark. Not that all such patronages are purely social- House Thrax and House Redrain in particular tend to have more martial associations, with common born great champions often being proteges, and sometimes is a sign that one might even be worthy of being raised into the peerage and granted land and titles. More often than not, a protege is not from the lord's own fealty, and it is customary for high lords of other houses to see such claims on their commoners or lower nobles as a flattering sign of respect, but exceptionally acrimonious relationships between two houses can lead to one immediately assuming disloyalty in a commoner willing to associate with an enemy.
Much of protege's social standing comes directly from the patron that chooses to honor them in such a way, and while a patron can have several proteges depending upon their social standing, no protege may claim more than one patron (and switching patrons is looked upon with askance, and may carry a degree of social opprobrium if most peers find it too self-serving or unjustified). Nonetheless, more than one commoner acting as a protege for a highlord has found themselves treated with the greatest of respect from the high born, and more than one lord has found their reputation in tatters after their protege brought social humiliation on their name by countless scandals while representing their lord.
In OOC terms, patronage is an alliance between any higher social rank character and any lower social rank character. The protege inherits 10% of the total prestige of the patron as long as their patronage relationship exists, and any prestige gain or loss has 10% of its value shared by the other party. This can represent any form of public alliance.