In heraldry, a crescent is displayed with the horns directed upward and is often used as a mark of cadency to distinguish a second son and his descendants. 

Gules is the heraldic name for the colour red, and it ranks highest among the colours. It is indicated in seals and engraved figures of escutcheons by parallel vertical lines. Because of its heraldic connection, the word gules is used poetically for a red colour or that which is red.

In the ancient Carolingian bloodline the Setons were senior representatives through their descent from Lambert de Lens, 2nd son of Count Eustace I of Boulogne, of the five Eustace fame. Hence the meaning of the three crescents in the Seton Arms, signifying descent from the second son of the House of Boulogne.

Adding to this Royal connection, Lambert married as his second wife Adele, sister of Duke William of Normandy, later to become William I of England. Lambert's father, Eustace I's descent was from the Counts of Ponthieu, who came from Berthe, daughter of Emperor Charles I,  or Charlemagne.  Eustace's wife was the heiress of Charles, Duke of Lower Lorraine, the last direct male heir of Charles I. 

Both Eustace I and his wife were descended in a multiplicity of the male and female lines of the French dynasty known as the Carolingians, descended from Charles I, King of France and Holy Roman Emperor who died in A.D. 814.  Seier de Seton (de Lens, in Flanders) was the eldest son of Count Lambert de Lens.  He founded the family of Seton and was granted lands in East Lothian by King Malcolm III, which were later named after the family's estate holdings in Northumberland named after the sea-town of Seaton-Staithes, later to be known in Scotland as SETON.

Later, Lambert's daughter Judith's second marriage, was to David I as his second wife, and Seier and Walter de Lens/de Seton were her half-brothers.  From King David I, Judith was the mother of Scotland's Queen Maud.  Later further adding to the family's royal blood and ties, Sir Christopher Seton married Lady Christina Bruce, sister of King Robert I, giving his Seton descendants further ties to the Royal Family of Scotland.  A double tressure was added to the Seton Arms after 1314 by King Robert I to recognize the Setons Royal lineage.