In 2019, we introducedFlame Retardancy Index(FRI) as a universal dimensionless index for the classification of flame-retardant polymer materials (Polymers,2019,11(3), 407).FRIsimply takes the peak of Heat Release Rate (pHRR), Total Heat Release (THR), and Time-To-Ignition (ti) from cone calorimetry data and quantifies the flame retardancy performance of polymer composites with respect to the blank polymer (the reference sample) on a logarithmic scale, as ofPoor(FRI˂ 100),Good(100≤FRI˂ 101), orExcellent(FRI≥ 101). Although initially applied to categorize thermoplastic composites, the versatility ofFRIwas later verified upon analyzing several sets of data collected from investigations/reports on thermoset composites. Over four years from the timeFRIwas introduced, we have adequate proof ofFRIreliability for polymer materials ranking in terms of flame retardancy performance. Since the mission ofFRIwas to roughly classify flame-retardant polymer materials, its simplicity of usage and fast performance quantification were highly valued. Herein, we answered the question “does inclusion of additional cone calorimetry parameters, e.g., the time to pHRR (tp), affect the predictability ofFRI?”. In this regard, we defined new variants to evaluate classification capability and variation interval ofFRI. We also defined theFlammability Index(FI) based on Pyrolysis Combustion Flow Calorimetry (PCFC) data to invite specialists for analysis of the relationship between theFRIandFI, which may deepen our understanding of the flame retardancy mechanisms of the condensed and gas phases.