A4 : The A4 format (size 8.3x11.7 in) is well understand if i can say its of a company letterhead size or the size of a standard piece of paper used in xerox/photocopy. This size is useful for companies, business enterprises, and local administrations with a lot of information to share. It can be useful for school leaflets for example, and they can easily be folded, put in an envelope and sent by post.
A5 : The A5 format (size 8.3x5.8 in) is the standard flyer size used for promotional campaigns and marketing strategies, newspaper insides etc. Flyers of this size are a convenient size for distribution too.
DL : The DL format (size 8.3x3.9 in) is comparable to the size of an office envelope.