Physically, lens flare is generally an additive effect, but the important detail is that it should be added in linear RGB/exposure space, not in a gamma-encoded display space such as standard sRGB values.

“Lens flare” commonly includes:

• veiling glare: a more uniform added exposure that lowers contrast
• ghosting: distinct reflected shapes or artifacts from internal lens reflections

Both are forms of added light. So if you’re trying to model flare realistically, additive compositing is the right idea in linear space.

If a plug-in doesn’t look like simple RGB addition when you inspect pixel values, it may be working in a non-linear color space, applying tone mapping, clamping, blending modes, or other artistic adjustments rather than strict physical simulation.

So the short answer is: yes, flare is best thought of as added light, but only straightforwardly so in linear space.