Is logos a one-time purchase?

Could you elaborate on the licensing model for Logos? Specifically, I'm curious to know if Logos requires a one-time purchase, or if there are recurring fees involved. I'm considering investing in this software for its robust research capabilities and would like to understand the full cost implications before making a decision. Additionally, does Logos offer any updates or upgrades as part of the initial purchase, or do those come with additional charges? Clarifying these points would greatly assist me in evaluating the overall value of Logos for my needs.