In this situation, the 84 car definitely knows where the 55 car is. The spotter for the 5
Pre-plan contingencies such as what to do if a caution comes out and you need to make an adjustment. The spotter must, in most cases, eye the pit entrance to make sure pit road is open. He must also tell the crew what is happening with the car so they can be ready. And, he has to keep track of the pace car's location and relay that to the team so they don't go a lap down.
A flat tire going into Turn 1 may not get noticed by the crew, so the spotter can relay the situation to the crew so they will be ready. The driver may say, "I've got a tire going down," but not know which one. The spotter may be able to eye which tire it is so the crew can have the correct size and pressured tire for that corner ready.
On the restart, if the car behind is laying back and making a run at your first place position, tell the driver so he can make him lift and brake, then say "go, go, go" once the car behind has committed to lifting. Oh, the possibilities.
Spot like you are in the seat beside the driver. In my spotting for road racing, I had a great time. The Daytona Prototype cars run with the GT class that are a bit slower, so that there is always overtaking of these cars. At times you are clearing your driver so he can move over to set up for the next turn.
You are letting him know that ahead is a very slow car. This is important because the closing rate may be too quick for the faster car to avoid a collision. This holds true for circle track racing, too. If you are side-by-side with someone racing for position, you need to know about a slow car on the inside so you can, a) crowd the other car to make him lift if you are on the outside, or b) move the other car over enough to get by the slower car if you are on the inside.
Again, both cars see each other, but in this case, the blue car is the faster car passing
Know that you must "feel" when the driver needs to be cleared, just like if you where driving. Never clear too early, but don't hesitate either. Know when a driver needs the information and what information he needs. When your driver is passing a slower car on the outside down the straightaway, he needs to know exactly when he is clear so he can either take the normal line into the corner or stay up. It's either one or the other and if the information is delayed, he may lose valuable time if he could have taken the low line.
In this case, by all means, key the mic early and as soon as clear happens, say "clear." If it happens to be a sudden announcement, I usually say two words, "you're clear," so that if I cut off the first word, "clear" comes through.
There are a few advanced techniques you can develop and use when spotting. When you get comfortable, you can begin to look well ahead and watch other cars at times when your car is all clear. Let the driver know every time he is all clear. That gives him a chance to relax his guard a little until he reaches new traffic to reduce fatigue.
Watch for future conflicts developing and if need be, alert the driver to them. If two cars get to racing side by side up ahead, they may be slowing down and this may be an opportunity to be alert to an opportunity to pass both cars if they were to get together and move up the track.
Be on the alert for caution situations, not necessarily waiting for the caution to come out. If an obvious caution situation develops, tell the driver immediately so he does not get into it. The flagman may be looking in another direction, as often happens, and the actual caution may come out too late for your car to avoid a problem.
If a car blows a motor, tell the driver to "stay high in 3 and 4, oil on the track," so he doesn't go flying in there and then slide into the wall. This kind of knowledge is a bit advanced and only veteran spotters, like Reutimann, are good at it, but it will drastically help your racing program if you can develop a holistic approach to your spotting duties.
Good driver/spotter relationships often are a significant part of winning a championship. The longer you work together, the better it gets. Talk to other veteran spotters and let them help you to get better. These guys are going to be beside you at every race, and a sort of camaraderie can develop in many cases. Good luck and speak clearly.