Which form is accurately labeled a a1a2a3a4 ?

Paper, books or canvases of sorts are labeled in size with A1, A2, A3 etc (the bigger the number the smaller the object) a4 paper is the standard size

The answer would be online media like a social media. it does refer to forms of different media content created publicly by the end users since 2005 and would cover how people use social media.hope this would be of .