FileFormatNum questions

I have just started looking at code saving in vba and I am sure this will seem like a stupid question to a lot of you out there. After reading a response from Jerry Beaucaire to a saving question in a previous post and then reading some bits from Ron de Bruin can someone explain a bit clearer the difference between ".xls": FileFormatNum = 56 and ".xls": FileFormatNum = -4143.

I get the impression that you can only use 56 in 2007 upwards but if this is the case why would you use it instead of -4143 as this appears more flexible at first sight?

I am asking the question because at work there are various machines running from Excel 2000 to 2007 and I would like to know what is safe to run on all the pc's and if it is a program that we are only going to use on a 2007 machine are there any huge benefits to using certain coding?

Oa secondary note, is there anywhere where there is a comprehensive list of all the various codes and what version they are compaible with?