In the Marathon world, there are several different types of creatures running around. Most of these are aliens, but there are others such as Marathon Crew Members and Exploding Bobs. There are also different kinds of weapons on the ship.
After positioning an object on the map, a window appears asking you what kind of object youd like this to be (shown below). This same window is shown when you double-clik on an object's dot in the level's window.
 
The 'Monster Type' popup menu allows you to select which type of monster you want this object to be. The bottom half of the window represents the information for this particular type of monster for the entire level. They are as follows:
Initial count: On startup, the number of instances of this object.
Must be > 0 or none will show, even if an instance is created on the map.
Minimum Count: If this object reoccurs, the minimum number that can be shown at
one time.
Maximum Count: Maximum number of recurring objects.
Max Random Count: Maximum number of random ocurrances.
Recurrance %: Percent chance this object will return in a time frame.
This same window is shown for weapons. For scenery objects which don't reoccur, a different version of the window is shown:
 
When the window first appears, it shows the default instance of that object. Please note that some objects DO NOT work with the release version of Marathon. If you do select a bogus object, Marathon WILL CRASH...I guarantee it. Selecting 'Cancel' will revert the object to the type it was when the window was brought to the front.
Any object which, to my knowledge, crashes Marathon, will have a '*' next to it in the menu.
Here's an incomplete list of objects that will make the release version of Marathon crash:
