When documents were printed using metal type, each size of a typeface was created with subtle variations to ensure that the character and legibility of the typeface was retained throughout the range of sizes.
For example, the versions designed for smaller sizes typically have wider spacing, for legibility, whereas the versions for larger sizes have finer details and greater stroke contrast, for elegance.
With digital type we have the ability to scale a font to any size, but this doesn't take account of the appearance of type at different sizes.
Several type designers have therefore created digital typeface families with versions of the typeface designed for use at different sizes. These are called opticals, or optical sizes, to reflect the fact that they are intended for viewing at a particular size.
Although naming conventions vary, families with optical sizes typically include two or more versions intended for the following uses:
Caption: For legibility at between 6 and 8 points.
Small text: For text at small sizes of 8 to 11 points.
Body text: For the ideal reading sizes of 11 to 14 points.
Subhead: For setting short phrases between 14 to 24 points.
Display: For setting large headings above 24 points.
Poster: For the text in posters above 36 points.