Why are there 3 different types of modifier available (global modifiers, profile abilities & weapon abilities)?

In UnitCrunch, these are referred to as modifier "scopes". While very similar, each scope serves a slightly different purpose and is applied in a slightly different way. Note that different scopes have a slightly different list of conditions & effects available to them.

Global modifiers

• Scoped to the entire simulation irrespective of what profiles are currently selected.
• Global modifiers persist when you change the currently active attacker/defender profile.
• Good for simulating effects that are "external" from the currently active profiles. E.g. Stratagems, auras, army rules, detachment rules etc.

Profile abilities

• Scoped to a specific profile and so can only be active when that profile is selected.
• Must include a "Profile role" condition so that UC knows whether to apply the effect when the profile is selected to be an attacker or defender.
• Saved with the profile itself and so is always recalled whenever that profile is selected.
• Good for simulating effects that are bound in some way to the profile but not to any specific weapon they may have. E.g. Datasheet abilities, stratagems that apply to specific units for easy recall.

Weapon abilities

• Scoped to a specific weapon on a profile and so can only be active when that weapon is selected.
• Saved with the profile itself and so is always recalled whenever that profile is selected.
• Good for simulating effects that are bound in some way to a specific weapon. E.g. Weapon abilities or "USR"s, stratagems that apply to specific weapons for easy recall.
• Some modifier features can only be applied to weapon abilities as UC needs to know which weapon they should be applied to (on a multi-weapon profile). E.g. Re-roll limit (single re-rolls).