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Basically it's a 2x8 step sequencer, however if you look closely there are four outputs at the top part labeled trig 1, trig 2, trig 3 and gate (which is also a trig output but it's longer, however the functionality is somewhat the same as the other trigs).

Trig 1 & 2 and trig 3 & gate are grouped. You can create a basic 2x8 pattern by flipping the two rows of switches to the upper position (which sets a trigger/gate) or leaving it in the middle (creating a "rest"). Use the outputs of trig 1 for the first row and trig 3 for the second row. You can expand this by using the "rests" from trig 1 or trig 3 as another trigger for trig 2 or gate. This way you have two independent step sequencers (trig 1 & 3), but also two sequences which can have steps whenever the two main sequences have rests (trig 2 & gate).

For an example, let's only use the upper row of triggers. Patch trig 1 to make a sound (let's say a closed hi-hat) and trig 2 to make another sounds (let's say an open hi-hat). If a switch in the upper row is set to the "up" position, the module will send a trigger to the trig 1 output. If a switch in the upper row is set to the "down" position, no trigger will be send to an output. If a switch in the upper row is set to the "down" position, the module will send a trigger to the trig 2 output.
The relation of trig 3 and gate is equal to the relation that trig 1 and trig 2 have.

Also, since you mention that this is your first modular system; the A-155 doesn't seem to have a built-in clock, meaning you need external gates or triggers to advance the steps or reset the module to the first step. Also, the A-155 output seems to have an analog only output, meaning that the CV outputs (this is handled by the lower half of the module, CV output is usually note data) are not in any musical scale. To force the CV outputs to be in a musical scale, you need a quantizer or an oscillator module that has a quantizer built in.
-- dubstepjoris

Uh, nice! And I was awared already that... I better change the A-155 from my composition or at least to consider a most flexible option since it is quite simple and need of many other modules. Thanks!!