"User Agent" is the type of name our bots use to identify themselves when accessing a domain or host. Until recently, we used "OnPageBot" (compatible; Googlebot 2.1; +https://bot.onpage.org/) as our default bot name. As of June 2018, this will be changed to "RyteBot" - a new version often capable of accessing more sites than OnPageBot.
If you would like to toggle between bot names, just click the name of your project on top the right-hand corner of your logged in page, then navigate to Project Settings (the cog button). Scroll down and click Advanced Analysis > How to analyze > Analysis user-agent. You can select from the available bot names or manually enter the name of your chosen bot in the blank field. Then, start a new analysis to try out the bot.