My understanding is that a designer can buy RF (royalty free) images and then use them for as many projects as he desires (e.g., advertisements, brochures, etc).
ELs (extended licenses) are meant to be used for:
- reselling images on other products (mouse pads, coffee mugs, tshirts, etc)
- increased production runs (for magazines/newspapers with large circulations)
- web usage/templates
- additional license seats
but be warned that each site has its own way that they handle ELs.