The sendInterval controls how often state updates are sent for this object.
Unlike most NetworkBehaviour scripts, for NetworkTransform this is implemented at a per-object level rather than at the per-script level. This allows more flexibility as this component is used in various situation.
If sendInterval is non-zero, then transform state updates are send at most once every sendInterval seconds. However, if an object is stationary, no updates are sent.
If sendInterval is zero, then no automatic updates are sent. In this case, calling SetDirtyBits() on the NetworkTransform will cause an updates to be sent. This could be used for objects like bullets that have a predictable trajectory.