The TDC calculates only the gyro angle for the torpedo, which is the relative bearing (from your bow) that the torpedo must turn to, to eventually be able to hit the target with the specified initial range, (peri/Uzo) bearing, speed and AOB, and set torpedo speed. (though there is a bug with changing torpedo selections set to different speeds. the lead isn't recalculated)
As the torpedo leaves the tube it first goes a short straight path, before it starts to make the turn. The displacement of the torpedo as it finishes the turn causes the gyroangle to be different from the initial periscope/uzo bearing. This is taken into account by the TDC, mostly based on range and bearing.
Aside from that it transfers the set depth to the torpedo.
And you can add a bit of a fudge/spread offset o the gyro angle with the salvo angle dial.
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