1 | XCorr | Cross-correlation between calculated and observed spectra |

2 | Δ*C*_{n} | Fractional difference between current and second best XCorr |

3 | Δ*C*_{n}^{L} | Fractional difference between current and fifth best XCorr |

4 | Sp | Preliminary score for peptide *versus* predicted fragment ion values |

5 | ln(rSp) | The natural logarithm of the rank of the match based on the Sp score |

8 | Mass | The observed mass [M + H]^{+} |

6 | Δ*M* | The difference in calculated and observed mass |

7 | abs(Δ*M*) | The absolute value of the difference in calculated and observed mass |

9 | ionFrac | The fraction of matched b and y ions |

10 | ln(NumSp) | The natural logarithm of the number of database peptides within the specified *m*/*z* range |

11 | enzN | Boolean: Is the peptide preceded by an enzymatic (tryptic) site? |

12 | enzC | Boolean: Does the peptide have an enzymatic (tryptic) C terminus? |

13 | enzInt | Number of missed internal enzymatic (tryptic) sites |

14 | pepLen | The length of the matched peptide, in residues |

15–17 | charge1–3 | Three Boolean features indicating the charge state |