The Lookup Table
Each LUT has 4 inputs and one output. The output, at your option, can be registered. If the output is registered, you may also specify a reset value. These two extra features really mean that this is a full logic cell, but in the code they are simply referred to as LUTs.
Input 0 into a LUT is the least significant bit, while input 3 is the most. When programming a LUT, you should send the lookup values in order of LSB first.
Note that LUTs number 0-15 are routed to output. They are also the first LUTs for which you will send data.
Please look at the image below for further details. It is accurate to how the LUTs in the Tritoncore-I work.
