The difference
Primary and secondary, in plain terms
A primary claim
Links a current condition to something that happened during service an injury, an illness, an exposure, or the conditions of the work itself. The evidence has to reach back to your time in uniform.
A secondary claim
Links a current condition to a disability the VA has already service-connected. The evidence has to reach back only as far as that condition and to show it caused the new one, or made it worse.
The medical question is the same in both cases: is it at least as likely as not that one thing led to the other? That is the question a nexus letter answers, and it is why a secondary claim usually needs one the link between two conditions is rarely written down anywhere in your file.