This is the Cross of Burgundy (Spanish: Cruz de Borgoña, Cruz de San Andrés, las aspas de Borgoña), used by Spain 1506-1701 as a naval ensign, and up to 1843 as the land battle flag, and still appears on regimental colours, badges and shoulder patches. It was the principal flag that flew over Spain and its colonial empire in the New World until 1785, when a new flag was adopted. It was also the flag of the Seventeen Provinces of the Netherlands in the 15th and 16th centuries, which had been part of Burgundy.
There were many versions of this flag, but in its most simple form consisted of a red saltire (diagonal cross) on a field of white. Actually, the design was supposed to represent two crossed branches, the extension on either side representing bases of limbs which have been cut off, and a few Cross of Burgundy flags actually do show limbs. Variants of the Burgundy cross flag—principally versions with smooth-edged saltires—became widely used by the Spanish military on both land and sea.
After Castile and Aragon were united to form Spain, Emperor Charles V’s (King Charles I of Spain) royal banner was the country’s only flag. By 1520, Spain had adopted the Cross of Burgundy as the new national flag. The saltire design was a symbol of Philip I, Duke of Burgundy and father of Charles I, who became Spain’s king in 1516. From that point on different armies within his empire used the flag with the Cross of Burgundy over different fields.
The Seventeen Provinces arose from the Burgundian Netherlands (1384 - 1482), a number of fiefs held by the House of Valois-Burgundy and inherited by the Habsburg dynasty in 1482, from 1556 held by Habsburg Spain. The Seventeen Provinces formed the core of the Habsburg Netherlands which passed to the Spanish Habsburgs upon the abdication of Emperor Charles V in 1556. When part of the Netherlands separated to form the autonomous Dutch Republic in 1581, the remainder of the area stayed under Spanish rule until the War of the Spanish Succession.
The emblem has been called in Spain “cross or saltire of Burgundy”, although the term “cross/saltire of St. Andrew” has also been used by those who have argued that St. Andrew is the patron saint of the Spanish Infantry. The patron saint of the Spanish Infantry, however, is not St. Andrew but Our Lady of the Immaculate Conception. The flag was first used, not by regular infantry, but by the equivalent of the present Spanish Foreign Legion, the “Tercio de Nueva España,” volunteer expeditionary troops, including infantry and cavalry, in Europe, Africa and the New World. The Burgundy Cross is nevertheless related to St. Andrew, not through the patronage of a Spanish branch of the armed forces, but through its Burgundian origin – St. Andrew being the patron saint of the Duchy of Burgundy.
The Cross of Burgundy has appeared throughout its history, and continues to appear at present, on numerous flags and coats of arms of bodies having no connection to each other—in various colours and in combination with other symbols. Users mostly have some direct or indirect relation to the historical Burgundy.
This is why I wear this Cross of Burgundy shoulder patch on my fencing jacket; because it was historically the flag of Spain, the HRE and the Lowlands as well as the symbol of the Tercios deployed in Europe, considered the best infantry at the time. I think this makes more sense than wearing a contemporary Spanish flag.