Definition
Nested sizes are several sizes of a pattern printed on top of each other, each with its own line style, so you can trace or cut the one that fits.
Most paper and PDF patterns are nested: the outlines of all sizes sit inside one another like contour lines, sharing grainlines and notches. A key on the instructions shows which line style or color belongs to which size.
The advantage is flexibility. One sheet covers a whole size range, and you can blend between sizes where your measurements fall in different ones, for example a size 12 at the bust tapering to a 14 at the hip. Draw the new line smoothly between the two over several centimeters so there is no step.
The drawbacks are practical. The lines can be hard to follow where they cross, and cutting out one size destroys the others. Many sewists trace their size onto pattern paper first, with a pencil or a tracing wheel. PDF patterns often have layers, so you can print only the sizes you need: check your PDF reader's layers panel before printing.
A pattern drafted for one set of measurements, standard or your own, has a single outline per piece and no nesting at all. That is the case with Sewly's patterns, which leaves nothing to trace or blend. For how size ranges are built, see pattern grading; for cutting accurately, see cutting line.