A more thorough explanation:

Senior party is a term used in intellectual property law to refer to the first person or entity to file an application for legal protection of a property, such as a patent or trademark. This means that they have priority over any other parties who file later.

For example, if two inventors come up with the same idea and file for a patent, the senior party is the one who filed their application first. They will have priority over the other inventor, even if the other inventor came up with the idea first.

However, being the senior party does not automatically guarantee legal protection. Other factors, such as the date of conception of the invention and the inventor's diligence in reducing the invention to practice, may also be taken into account.

In a patent interference proceeding, the burden of proof is on the challenger to prove that they are the true inventor and should have priority over the senior party.

Overall, being the senior party can provide a significant advantage in intellectual property disputes, but it is not a guarantee of legal protection.