Hi sir
300 Indicates the particle size (particle size in microns). The higher the number, the coarser the powder.
25C designates a specific characteristic of the powder (composition, coating type, or other technical specification).
500 indicates a finer particle size than 300, meaning a smaller powder suitable for prints requiring less relief.
15C Fine Indicates a finer particle size (15 microns) and a specific powder type.
For offset printing, the choice depends on your print media (coated paper, cardstock, etc.) and the desired finish. A finer powder (500 - 15C Fine) is often used for prints requiring less relief and a better finish, while a coarser powder (300 - 25C) is ideal for printing on thicker media where faster drying is required.
Good luck and be welcome