For print.pp3 the value is NULL.
For unitname.pp3 an object of class "units".
Details
These are methods for the generic functions
print, summary,
unitname and unitname<-
for the class "pp3" of three-dimensional point patterns.
The print and summary methods print a description
of the point pattern.
The unitname method extracts the name of the unit of
length in which the point coordinates are expressed.
The unitname<- method assigns the name of the unit of length.