Dry, wet and free voltage contact?
I haven't heard all of those terms in use -- they may be part of a local slang. Alas, the main problem with contacts in connectors and relays is that the metal coatings/platings on the mated contact pair differ depending on the application and current.

The main choice is gold vs. silver. If you have low current (up to dozens of mA) signal connections, you need gold plating/flash. If you have high current connections, the gold flash will be eroded if the connection is made or broken under load. Such connections are from that point onwards unsuitable for low current use! The ideal high-current plating material is silver.

Tin contact plating is not something I'd really like to use in an application designed for longevity.