"1 year with possibility of extension" - what does this mean? When looking at adverts for postdoc positions, I often see examples from the US that say something like "the position is for one year, with the possibility of extension for another 1-2 years".

Literally, and contractually, the meaning of this is obvious, but I am interested in guidance as to whether it consistently means anything in practice.

For example, I can imagine that it could mean "I only have funding for one year, and I'm hoping the successful applicant will find themselves funding to stay longer". But I can equally imagine that it is "I have funding for three years of a postdoc, but I want to see if I like them before committing".

Is there a "usual" subtext? Or is it not possible to read anything into this?

When considering an intercontinental relocation, it would be helpful to have some idea of how likely the longer duration is!

It's in a scientific field, in case that's relevant.
I'd probably read it this way: 


  I have funding for one year for sure, and hopefully for additional years, but it might depend on the success of future grant proposals, so I can't make any promises.  But assuming that my funding is successful, and the postdoc is continuing to do a good job, I'd be happy to keep them for 2-3 years.


In some cases, postdocs might be funded by "hard money" coming from within the university; this is common in math, and such positions often have names like "Famous Person Assistant Professor".  In this case, funding can be assumed to continue (barring a financial catastrophe for the university) and it means more like "2-3 years assuming you do a good job".