Attachment Points
Some equipment won't fit inside your pack. Tents and sleeping pads are often attached on the outside of the pack, and there may be other equipment you'll want to carry outside as well, such as crampons (for icy conditions) or snowshoes. Obviously, you need attachment points on the pack to carry these items. On EF packs, there is sometimes enough room to lash these items directly to the frame. On IF packs, attachments are located on the pack itself.

Nylon webbing (daisy chain) is also frequently sewn onto packs to create attachment points. These methods require that you carry extra straps to attach your equipment to the lash points. Other methods include criss-crossed perlon rope that can be tightened down with a spring-loaded nylon lock. (These attachments are more suited to items such as crampons, unless you feel you can securely attach a tent without the concern that it will slide out the bottom.) And there are other variations such as pockets that can be folded over equipment and secured with quick release buckles.

The point to be made is that you need to think about what you want to carry on the outside of your pack and then look at the attachment points available on the pack you're considering.