I noticed during my application process for post-doc jobs that several say "1-2 year appointment" or "possible renewal for second year". I understand from this answer that it is in everyone's benefit to not be "stuck" in a postdoc situation for too many years for fear of starving oneself.
My question is, if not mentioned, what is the general criteria for a renewal of postdoc appointment to a second year (or perhaps to a third)? Most of the applications I have put in to labs/univs. are not clear at all on renewal criteria. Is it based on need? performance? Mutual understanding? Does anyone have examples on some of these situations?