First, I would say that it is realized PnL because with futures, you always have to settle up at the end of the day in the margin accounts. If you bought the futures at 98.51, then you only post margin since the futures contract has zero value. If the contract settled at 98.505, then you lost 0.005 on the contract. Each Eurodollar contract is on 1MM notional, but over the 3M period, it is like a 250K notional. The payoff is 2,500 per point per contract, so you have a final payoff of 250 * 2,500 * (-0.005) = -250 * 12.50 = -3,125. The 250 factor is the number of contract you referenced in your question.
It is worthwhile to think of a futures contract as a series of one day forward contracts that get settled up each day in your margin account and you have the option of exiting at anytime. This way of thinking can help to understand an important feature of the Eurodollar contracts - i.e. the convexity correction that helps to convert futures prices to forward prices and vice versa. This is a feature of any futures contract, but is most pronounced and studied for Eurodollar contracts since they have expiries out to 10 years and the convexity correction is bigger for longer dated contracts.