Kem,
LOL, when have you ever seen any rule at Disney 100% enforced?
But since you asked for the "skinny" on the UMP here you go, warning it is kind of long.
Ultimate Park Hopper Passes - Provide unlimited admission to Magic Kingdom, Epcot, Disney-MGM Studios, Animal Kingdom, River Country, Blizzard Beach, Typhoon Lagoon, Pleasure Island, Disney Quest and Disney's Wide World of Sports (not valid for Hard ticketed events).
There are major advantages and disadvantages to purchasing UPH's. The major advantage is that you can come and go as you please to any of Disney's theme parks, water parks, Pleasure Island, Wide World of Sports and DisneyQuest for as long as you stay without worrying about keeping track of used park days and plus options as you would with regular tickets. The major disadvantage to UPH's is that if you don't go to any of the Disney parks on a certain day (due to illness, offsite activities, visits to other Orlando theme parks, day spent at the resort pool, etc) you are still paying Disney for a day of admissions that you didn't use and can't save for future use.
UPH's are valid from the moment you check-in (or 12:01 am of the first day you purchased them for if you dropped your arrival day) through midnight of your day of departure day (or through midnight of the last full day you purchased it for if you dropped your departure day.
Ultimate Park Hopper Passes are sold two ways: as a package (either included or added) with your WDW room reservation through CRO or WDTC or as a counter sale you purchase upon check in at your resort front desk. The two methods have different restrictions.
When you purchase your UPH's through CRO/WDTC, they are directly tied to the entire length of your reservation and the number in your party; hence the length of stay pass must be purchased by all members of your party and must equal the number of nights reserved. Holders of valid AP/PAP's are exempted from this requirement. You must purchase UPH's to cover your arrival day straight through to your departure day. One advantage to purchasing this way is you will get a slightly reduced advance purchase price for each UPH you buy.
If you wait until resort check in to purchase your UPH, you will pay a little more for your UPH's but it may be more advantageous to you since you may have either your arrival day or departure day removed thus saving you some money over the advance purchased UPH's. If your UPH's were purchased as part of a Disney package (Resort Magic, Grand Plan, etc), you cannot change them to drop a day.
TIP: Resort front desks can sell different tickets to different people right on the Resort ID card. In addition, each person does not have to buy a ticket. They can customize each person's ticket to their specific needs.
If you are staying at 2 different WDW hotels, you would have to purchase 2 different Ultimate Park Hopper Passes, not one for the entire visit on property. It will cost more getting two shorter UPH's than one longer one. Also, if staying for 5 days, you may not purchase a Ultimate Park Hopper Pass for 4 or less days. However, I continue to hear conflicting information regarding this so be sure to check with Guest Services when you arrive.