What is "4-state binary radix" in Modelsim

Hi, For Modelsim XE 6.1e, it says the following on page CR-27 of command reference. I do not understand what is the "4-state binary radix". Could you tell me what it means? Thanks a lot.

Searching for binary signal values in the GUI When you use the GUI to search for signal values displayed in 4-state binary radix, you should be aware of how ModelSim maps between binary radix and std_logic. The issue arises because there is no "un-initialized" value in binary, while there is in std_logic. So, ModelSim relies on mapping tables to determine whether a match occurs between the displayed binary signal value and the underlying std_logic value.

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