When your artwork contains transparency, Illustrator performs a process called flattening before printing or saving the artwork. During flattening, Illustrator looks for areas where transparent objects overlap other objects and isolates these areas by dividing the artwork into components. Illustrator then analyses each component to determine if the artwork can be represented using vector data or if the artwork must be rasterized.
In most cases, the flattening process produces excellent results. However, if your artwork contains complex, overlapping areas and you require high-resolution output, you can control the degree to which artwork is rasterized.