If you specify a name, Hudson will assign a unique port number as an environment variable. This is preferable as it lets Hudson manage TCP port allocations. To avoid platform specific issues, all upper case names are recommended.
If you specify a numeric value, Hudson will make sure that port number is reserved while a build is in progress. As a result of this, builds may block while another build that uses the same port number is in progress. This is useful when your build hard-codes a port number and you cannot change that.