Altitude is measured from the sea level and is expressed in meters. By sending info to the server user can find out some information about the current state of the program. Using some planetarium type of program which often also includes a rather large asteroid database or to subscribe to the Computer Service of the Minor Planet Center is the usual way to handle this problem. Both approaches have a disadvantage of being rather slow because it is necessary to type coordinates and time into some type of form. By using one of these programs or a custom written program which makes use of the check command the task of identifying asteroids becomes very easy - only one click of a mouse or running a script which identifies all known asteroids imaged in one night.
Distance is expressed in arc minutes.
Most of the measured positions of known asteroids will be within 1 arc second from the calculated positions but giving the distance of 1 arc minute will allow identification even for times outside the epoch of validity of orbital elements while keeping a probability of having two asteroids inside this circle rather small. Of course, if you want to list all asteroids within a wider circle, it is also possible. The largest distance allowed is 5 degrees. Magnitude simply sets the maximum magnitude of the asteroids which are listed as a result. If the asteroid is not numbered, the number is 0.