Basic questions on Poloniex

Hi, I have a few questions that I can't seem to find answers elsewhere. Would appreciate if someone can assist me.

Price and volume change on Poloniex is with respect to which exact timing of the day? ( For normal stock trading, change is with respect to closing vol and price of the day, however for crypto the exchange runs 24/7 so I am a bit confused).
What is the difference between placing a stop-limit order and sell- buy order. Example: Price is at 1.5, I put up a buy order at 1.3. The order would stay there until I am able to buy my coin at 1.3. Apparently limit order works the same way so what is the difference?
What are the requirements for a coin to be listed on Poloniex? How is it announced and how often do exchanges list new coins, as I have not come across any new coins on Poloniex recently at all.
What happens if a coin's trading volume is consistently low? Would Polo take them out eventually? If yes does it mean a total loss in your investment as some coins cant be withdrawn and deposited across exchanges.

Cryptoflame