No problem -- we're actually formalizing some of this language in a
separate document so your comments are welcome.
The source code is just another name for the computer program. FDS is
a very large fortran program.
When we fix a bug, we are ensuring that the program is doing what it
was designed to do. For example, the user's guide might explain how to
output a certain quantity, but when the user tries to do it, that
quantity might show up as zero, or -9999, or whatever. This is clearly
a bug where the program contains an error that somehow over-writes the
computed value of the quantity. We fix it, and then release a
maintenance version. We then encourage everyone to upgrade to this
because we don't want anyone to think that the output quantity is zero
when it is not.
A change in functionality might be where we decide that a particular
default value should be changed to something else, or where we decide
to use a different algorithm for, say, smoke detector activation.
These types of changes require a minor release, because the model user
ought to be aware that the mathematical model has changed slightly.
Here, you might want to not upgrade if you are right in the middle of
something. However, you don't want to stay too long at a particular
version because we do not make bug fixes to older versions.
Hope this helps.