irun/ncsim/ncvlog SystemVerilog support options

I see the following options for SystemVerilog in irun/ncsim/ncvlog. Why are there two options for SV 2005 and 2009.

What option to use for the latest SystemVerilog 2012(IEEE 1800-2012 SystemVerilog) standard?

-SV -- Enable SystemVerilog features
-SYSV05 -- Enable SystemVerilog features with only SV-2005 and earlier keywords
-SYSV09 -- Enable SystemVerilog features with only SV-2009 and earlier keywords
-SYSV2005 -- Enable SystemVerilog features with only SV-2005 and earlier keywords
-SYSV2009 -- Enable SystemVerilog features with only SV-2009 and earlier keywords

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