This is going to be more useful for scenarios involving stryker missions. Even if you encounter a small unit of the planets overall defense forces, this will give some information on their quality and equipment. And for non-Styker campaigns, the GWP (Gross World Product) and military strength gives an idea of a planets power and influence that they have over neighboring systems. The unit specifications are vague. an infantry unit could be a foot soldier or a robot. A mechanized unit could be a tank, armored air raft, or a 50 ton robot. On low techlevel planets - a mounted troop is calvary. The referee can have some latitude depending on the tech level of the planet.
Using wikipedia as a reference the GWP per person is calculated. It takes into account whether a planet has agriculture, manufacturing, etc. Then once the GWP per person is calculated then the total gnp is calculated. The biggest variable is the population. It is always a random roll. So a given planet can have a different GWP each time because of this.
Again using wikipedia as a reference, a number of factors were used to determine the quality and quantity of units. Dictatorships tended to have higher quantity, but lesser quality of military units. The Democracies tended to be the opposite. Manufacturing economies had better quality units than non-manfacturing economies and so forth.