Spot colour is the most commonly used screen print separation.

and typically consists of well defined seperate areas of colour or half tone. There is sometimes a need to create overlaps where the colours meet when printing in plastisol inks, this is called trapping and is generally created by using an outline to the value of 0.5 in a vector based programme such as corel or illustrator. When dealing with raster artwork (pixels) then a value of 1 or 2 pixels is normal. Advertees favour the use of water based inks, these inks require a different kind of separation where the colours meet and fit exactly with no overlap, this is called butt registration and obviously requires an accurate press. Most T shirt printers have there own spec for separating artwork preferring to be supplied original artwork files. Always ask your screen printer what they require if you want to create the separations yourself.