Sentence:  There were all kinds of different tips with varying widths of sprays—fat and skinny ones with special names. Fats were for <WSD>fill-ins</WSD>, and skinny tips usually for the outlines, unless you used a fat cap for a tag and attempted to “flare” the ends of your letters. 
Meaning of the word marked with <WSD> tags:  Something added to fill a gap.