Vacuum x Vacuum full

I would like to know if space disk is free to the Operational System
during a standard vacuum (without the full option)?

 

I found in section "21.1.1 Recovering disk space" of Postgresql 7.4.2 manual
something about it.

"The standard form of VACUUM is best used with the goal of maintaining a fairly level steady-state usage of disk space.The standard form finds old row versions and makes their space available for re-use within the table, but it does not try very hard to shorten the table file and return disk space to the operating system. "

 

Even so it is not clear. They say the space will be available for re-use
within the table so I realized the space will not be available to
the operational system. But they also say the command "does not try very
hard to shorten the table file and return disk space to the operating
system" what means to me that it could sometimes free space to the
operating system.

 

I could not clear my doubt. Will the standard form, free disk space to the
operational system or not?

 

I did some tests and discovered that the standard form frees disk space
to the operational system. But if the standard form already frees it why the full form
will do different?

Thanks in advance!