An idea is rationally true when it is logically consistent inside a rational framework. It does not need to be directly measured like an empirical claim, but it must hold together. It must make sense within the system that supports it.
Math is the clearest example. Natural numbers are rationally true within a mathematical framework. You do not need to find the number three under a rock. You understand it through logic, relation, and structure. The same is true for many ideas in ethics, law, politics, and philosophy.
But rationally true does not mean magically true in the material world. A political theory can be coherent and still fail in practice. An ethical rule can be logical but too rigid for real life. When a rational idea makes empirical claims, those claims must be tested.
In TST, rationally true ideas matter because human beings think through frameworks. We need logic, categories, values, principles, and models. They do not replace empirical truth, but they help organize it.