The three levels
99% modelling quality: uses real tick data from the broker history server. Every price movement is replicated as it occurred. The only reliable data source for scalping and grid EAs.
90% modelling quality: uses OHLC bars with interpolated tick data. Adequate for daily-close systems but can misrepresent intra-bar fills by 5-30%.
25% modelling quality: lowest quality, sometimes used for rapid parameter scanning only.
Practical note
Ask for the specific data source and date range when evaluating any EA backtest. “99% modelling quality on XM real tick data from 2020-01-01 to 2025-12-31” is a verifiable claim. “Excellent backtest results” is not.