The Basic Distributor trigger is a very simplistic decoder that expects input like what a traditional distributor outputs. That is, 1 pulse per cylinder per cycle.
For this reason the signal lacks any cylinder position signal and so without a missing/added tooth or camshaft signal reference Speeduino cannot calculate crankshaft angle, phase of cycle, or cylinder assignment. A distributor must be used to route the resulting sparks to the correct cylinders (With the exception of single cylinder engines).
The signal can be as simple as the breaker points from an old pre-electronic distributor, to a crankshaft wheel without any abnormal, extra, or missing slots, provided it is conditioned appropriately to 0v-5v. Most who have installed aftermarket tachometers are familiar with the simplicity of the signal with the only variation being the number of pulses in each crankshaft rotation.
By its nature the resolution of a simple distributor wheel is quite low. The exact resolution depends on the number of cylinders (The more cylinders, the higher the resolution), but even with an 8 cylinder engine there are only 4 pulses per revolution. This can impact timing accuracy if running ignition control from Speeduino and in most cases upgrading to a higher resolution trigger wheel is strongly recommended.