requesting help with Variable OSREL in port Makefile

[qjail1]

 >> .if ${OPSYS} == FreeBSD && ${OSVERSION} < 1000000
 >> IGNORE= builds only on 10 or greater than 10
 >> .endif
 >
 > That's OSVERSION. The OP asked about OSREL.
 >
 > (however, OSVERSION is more useful as one can test it numerically; it
 > is documented in Chapter 16 of the Porter's Handbook, if the OP wants
 > to know more)

I read Chapter 16 of the Porter's Handbook before posting. It doesn't 
say how you got to a "1000000" value for OSVERSION. Why so many zeros?

A 10.0 value is what I interpret to be the value for OSREL. Is that 
correct?

The IGNORE= keyword is confusing also. The port will build/install on 
all versions. Its just that some things in 10.0 changed that caused the 
port source to be modified to work on 10.0 and newer systems. It will 
not work correctly on 9.3 and older systems. So I want the port/pkg to 
check the host version and refuse to install if installing on 9.3 or 
older systems. This has no impact on the auto building of pkgs.

The Porter's Handbook section containing the OSREL Variable talks about 
the placement of the OSREL Variable is critical and based on where 
bsd.port.pre.mk statement is located in the Makefile. I don't have a 
existing bsd.port.pre.mk statement in the Makefile. Do I need to add one?