In Stratix® II and Stratix III devices, each LAB is composed of ALMs, and each ALM can usually implement 2 ALUTs. However, in some cases, only one of the ALUTs can be used, making the other one unavailable. Unavailable means that the ALUT is not currently implementing any logic for the design, but it is not available to be used.
The number of ALUTs Used reported in the Quartus® II Fitter Resource Usage Summary is the number of ALUTs that are used to implement logic in the design. This report does not include unavailable ALUTs; unavailable ALUTs are reported separately.
The number of Combinational ALUTs reported in the the Quartus II Fitter Utilization by Entity includes unavailable ALUTs in the total. This number equals the sum of the used ALUTs and the unavailable ALUTs reported in the Fitter Resource Usage Summary.