Organisation = How long your troops can fight effectively in single engagement before they automatically withdraw and retreat to another province. You can issue manual withdraw for a stack but there is a delay before it can be issued.
Morale = How fast troops regain organisation.
Tactics = How much your troops take casualties besides their raw stats, so higher tactics value -> less casualties.
Discipline = Extra discipline behaves like tactics.
Support = How well the unit operates in support role, aka in the 2nd battle row. The battle row setup is automatic as noted in earlier posts.
Recon = Speeds up occupation and also reduces defender's dig-in bonus when attacking dug-in stack.
Width = How much space they occupy in combat.
You can see province supply limits vs supply burden by selecting a stack and hovering mouse over their destination. If you go over limits you will take extra attrition. Sieging\occupying a province etc will also have fixed base attrition rate, and all attrition have a cap so even if you organise comic supervillain 500k stack in a single spot they will not perish instantly, but needless to say that's waste of manpower in most cases. There are also diminishing returns so three hussar brigades with high total reconnaisance won't occupy a province faster than hussar brigade and some infantry would, and large bulk of infantry occupies faster with recon elements than without etc.
Or roleplay?