Truck Annual DOT inspection

I base in Texas and have seen that most trucks that have a USDOT number have a Texas State Inspection sticker in the window and not the DOT annual sticker on the exterior of the cab. The requirements for a DOT annual and the state inspection are about same but, when is the DOT annual inspection required (most are near the exterior drivers cab door) and what is the difference between the two inspections. In TEXAS the state inspection usually cost about $62.50 and the DOT cost about $40.