Return values are shared between all procedures and have always the same format:
ReturnCode, Info= function()
which is generally rendered by many programming languages as an array of two elements.
The first one, ReturnCode, is always an integer. In particular, for succesfull operations, and only for them, ReturnCode is zero.
The second element, Info, changes depending by the called procedure.
When ReturnCode is less than zero, something wrong happened. In that case, info is a string and contains a Human Readable Error: this way you can better understand what happened.
When operations are succesfull, Info basically depends from a simple condition: does the called function need to return something? If the function does not need to return data (for example functions used to update availability), Info is generally a string, like Ok.
For functions returning data (for example functions that return reservations), Info can be an Integer, a String or even a Complex Structure (like arrays or KV structs): check the An introduction to Wired section for more info.