Sometimes you lack the skill needed in a certain situation. In these instances, characters may default their skill test to the linked aptitude. This reflects the fact that most learned skills are developed from some sort of baseline physical ability. Even though you may not know how to do something, you’ve likely seen how it’s done at some point, have some idea of how to do it, or can at least take a shot at it. Naturally, you’re not as good as someone who has training in that skill, but it still allows you to make an attempt.
Not all skills can be defaulted. Some skills are simply too complex or obscure or demand special knowledge or ability for someone to attempt untrained. For example, brain surgery or most psi skills are simply beyond anyone who doesn’t have that ability or the knowledge of what they’re attempting.
Defaulting to Field Skills
In some cases, a character may not possess the particular Field Skills that a test calls for, but they may be skilled in another related field. For example, a test to conduct an alien autopsy might call for an Academics: Astrobiology roll, but a character who doesn’t have that skill may be allowed to default to Academics: Biology instead. The gamemaster decides if and when to allow this, perhaps applying a modifier to the test based on the difference between fields.
Defaulting to Related Skills
If the gamemaster allows it, characters may default to a related skill that also has some relevance to the test at hand. For example, a character skilled in Kinetic Weapons might not be trained in the use of a laser, but they know enough to point at the target and pull the trigger. Likewise, a character might not be skilled in Investigation, but the gamemaster could still allow them to use their Perception skill instead in order to realize that a body had been moved from the place where it had been shot. In situations like this, when the gamemaster allows defaulting to a related skill, a –30 modifier should be applied to the test.