Naisargika Bala is the natural strength of the planets. This strength does not depend on the specific chart but is fixed for all charts.
Planets with nearly identical Shadbala can be evaluated by their Naisargika Bala, i.e. the planet with higher Naisargika Bala can be regarded as stronger.
The seven bodies are ordered by their luminosity and get relative strength according to their position in that order. Sun has the highest luminosity, Saturn is the darkest planet; other planets get portions of 1/7 Rupa according to their position in that order.