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Reticules (reticles) are normally used to display range of weapons or building placement. However, this can be expanded to show the radius of auras and other things. Vanilla examples would be viewing a sniper's range or a grenade's radius.
You may view the different types of reticules in the ui_reticule category in attribute editor. From these, there are several different types of reticules located within the data extension. There are a limited amount of textures being used and unfortunately most of them are either green or red. However, we can recolor some of them as well as pull textures from other ui categories probably. You can also try to recolor the green/red types of textures but you wont be able to get the full color palette as they are already pre-colored. You may also use the textures from the other ui categories as these are usually meant to be recolored.
Reticules can be used through the reticule action, the ui_reticule in abilities, or ui_range in weapon.
The greatest potential of reticules would be to add custom textures which will effectively allow us to paint directly on terrain. Unfortunately I haven't been able to find where these are located or the dimensions needed for the texture file. Maybe someone else knows.
The other types of UI categories are much simpler and self-explanatory in what they do.