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From Froggie's excellent guide

1.Disciplined means the unit is less likely to chase off after enemy units when it is in the impetuous morale state. It also means that the unit doesn't care about routing units on its own side unless they are of disciplined or elite status. The death of your general also has a reduced effect on disciplined units morale. This means that routing peasants and the like can no longer panic your army. If a unit does not possess disciplined status by default the only way to add it is to build the military academy in one of your production provinces. This will not add the disciplined tag to your unit but it will increase their discipline slightly. Units like knights still tend to be impetuous even with the discipline upgrade.

When a unit is placed in Hold Formation the men tend to stay in their place in the unit formation, on engage-at-will they break ranks and 'swarm', so they attack an enemy in their little space or sphere.

I'm pretty sure that discipline isn't a morale bonus, its somewhat different.

Hope that helps

ichi