People justify an answer already given to an argumentative question, while they deliberate on an open argumentative question, when they do not know its answer, either individually (Third parties, S. Role), or when the group has not yet reached a decision.
Deliberation takes place in a situation of doubt about what to do, while justification bears on a decision which has already been taken. The starting point determines the difference between justification and deliberation.

— Deliberation intervenes in contexts of discovery. It develops from argument to conclusion. A decision is to be taken, and I deliberate to construct it through an inner or collaborative deliberation; the arguments condition the conclusion. The argumentation:

Question: Should I resign?
[Deliberation: I weigh up the pros and cons] The answer states the conclusion: I resign.

— In contexts of justification, the discourse proceeds from conclusion to argument. I resigned, this is a practical reality:

Question: Why did you resign? Justify your decision!
Justification: I was sick and didn’t get along with my boss.

A decision has been taken, and, when required to account for it, I explain why I took this decision or made this choice, I recall all the good reasons I had to do so, and, if necessary, I invent new ones. Now, the conclusion determines the arguments.
Deliberation leads to a conclusion introduced by so, therefore; justification enumerate good reasons introduced by since.

The mechanisms of argumentation are valid for justification and deliberation. I deliberate, I reach a conclusion and make my decision. When I am asked to justify this decision, the same arguments, which were deliberative, become justificatory, and explain the decision taken, S. Explanation.

In the case of deliberation, there is real uncertainty about the conclusion, which is constructed in the course of a cognitive and interactional argumentative process. In the case of justification, the conclusion is already there. Justification tends to erase doubt and counter-discourse, whilst stimulating deliberation.
Private arguments put forward during an inner deliberation may have nothing to do with the arguments put forward publicly as a justification for the same conclusion, S. Motives and Reasons.
Situations of pure deliberation and pure justification are border cases in which I do not know what I will conclude or do (full deliberation), and I’m sure I did well (full justification). The same arguer may oscillate between justification and deliberation, for example if, during the justification, he or she questions the decision already taken, or is about to change his or her mind.
If we postulate that any argument that presents itself as deliberative is in fact oriented by a decision which has been taken unconsciously, then anything and everything is in fact justification. Yet the institutional organization of debates reintroduces deliberation. A debate may well be deliberative when each of the parties comes with firmly entrenched and duly justified positions and conclusions. The shock of justifications produces deliberation.