What does a nominal rating indicate about a filter?

A nominal rating for a filter typically refers to the size of particles that the filter can effectively block, indicating that it captures particles that are at or below a specified size. In this case, the correct answer states that it blocks half of the specified size of the particles. This means that if a filter has a nominal rating of, for example, 10 microns, it is designed to capture particles that are approximately 5 microns or larger, signifying a general measure of its filtration capability.

This concept is important in understanding how filters function within operations, especially in contexts like air or liquid filtration, where the size of contaminants is critical for system performance and reliability. A nominal rating allows users to assess the expected performance of a filter based on the particle sizes which might be present in the environment it is being used in.

The other options provide misunderstandings of what a nominal rating signifies. Rather than quantifying the percentage of particles blocked or combining measures of overall efficiency and size of particles blocked, a nominal rating specifically pertains to the size threshold below which a certain percentage of particles will likely be filtered out.