Attention, memory and perception
Chunking
Also called Miller's law
People remember information better when it's grouped into a few meaningful chunks, because working memory holds only a handful of things at once.
A long string of digits is hard to hold; the same digits in three groups are easy. Grouping turns many items into a few, and gives each group a meaning.
Break long content into short sections with clear headings, group related settings, and format numbers people need to read back, such as phone numbers and codes.
In practice
A bank shows an account number as 4012 8899 1234 instead of 401288991234, so people can check it against a card at a glance.
Common mistake
Treating 'seven, plus or minus two' as a design rule for menus. It describes memory for things people must hold in mind, not how many items they can scan on a screen they can see.
Where it comes from
George Miller's 1956 paper The Magical Number Seven, Plus or Minus Two described the limit and the role of chunking. Nelson Cowan argued in 2001 that the real limit is nearer four.