This is the first page of Stevin's Arithmetique, originally published in 1585, in which he gives several definitions. In particular, he argues, contrary to Euclid, that unity is a number and that "number is that which explains the quantity of each thing." Thus, "number" is not only a collection of units and, in essence, Euclid's distinction between "number" and "magnitude" (coming from Aristotle) no longer makes sense.
On this page, Stevin shows how to solve various types of quadratic equations. In his notation, the circle around a given number designates the unknown raised to that power.
On page 103, Stevin shows how to translate some problems from Diophantus's Arithmetica into his own algebraic notation and then how to solve them.
On page 209 of the Oeuvres, we find the introduction to Stevin's La Disme, his work of 1585 explaining how to use decimal fractions. Again, he uses circles around digits, but these now stand for the appropriate decimal place.