ruby on rails - subdomain-fu gem tld

what mean tld_size in subdomain-fu gem. example deafult having,

  subdomainfu.tld_sizes = {:development => 1,    :test => 0,    :production => 1}

in initializers/subdomain-fu.rb

i have debugged , found answer,

if config follows means,

 subdomainfu.tld_sizes = {:development => 1,   :test => 0,   :production => 1}

if url test.lvh.me:3000 means subdomain-fu gem's current_subdomain return test. if set tld_size below,

 subdomainfu.tld_sizes = {:development => 1,   :test => 0,   :production => 0}

then url test.lvh.me:3000 means subdomain-fu gem's current_subdomain return test.lvh

thanks...

KHS