If process is not supplied or is
nil, the current process (that is, the result of calling get-current-process) is used.
Note that removing a property is different from setting its value to
nil, because when process-property is called with a default for a property that was removed, it returns the default, but for a property that was set to
nil it returns
In the typical case when only the current process sets the property (even if other processes read it), private properties can be used, and are much faster in SMP LispWorks, because they do not need to deal with parallel setting. See process-private-property.
LispWorks User Guide and Reference Manual - 13 Feb 2015