I have a hard time understanding what it means.If Dynamic is unchecked, the first unit(s) or building(s) found to meet the indicated requirements will be chosen for the object definition and no further selecting will be done, even if the object definition is used in another condition or effect. Checking the Dynamic box means new units or buildings will be picked for the object each time the object is used (the same unit(s) may or may not be selected each time). This is useful if you want to use the same Object Specification in several condition and/or effect definitions but do not want the same unit(s) to be used each time.
I tested it with unit spawning. It seems that if the box is unchecked (i.e. static), and let's say you are spawning tanks, if you delete the first tank, then no tank will ever spawn again. But if you check the box on, then even if you delete all the spawned tanks, new tanks would still be spawned.
Can someone who knows more about this function give an explanation?