The short answer
Share of model is your brand’s portion of the AI-generated answers for your category, measured across engines like ChatGPT, Claude, Gemini, Perplexity, and Google AI Overviews. It is the AI-search answer to share of voice: when you run a defined set of category prompts, share of model is the percentage of those answers that mention or cite you compared with the competitors that appear alongside you. A high citation rate tells you how often you show up; share of model tells you how much of the conversation you own versus rivals.
Why it is read per engine
Each engine retrieves and trusts sources differently, so your share of model can be strong in one and weak in another. You might own a large slice of Perplexity answers while barely appearing in Gemini, usually because the engines reach different pages and weigh authority in their own way. Blending everything into one number hides that, so share of model is most useful broken out per engine, where it points you at the specific surface that needs work.
How it is calculated
Run a fixed set of category prompts against each engine, then for every answer record which brands are mentioned or cited. Across the whole set, your share of model is your count of appearances divided by the total brand appearances for that engine. Tracking it on a schedule turns it into a trend, so you can see whether a content push moved you ahead of a competitor or whether a rival is gaining ground.
See competitor AI visibility for how share of model is reported against your rivals, or AI citation tracking for the full picture. To get your baseline, run the free AI visibility scan.