What is a Type 2 and Type 4 indicator?

These are not standard classifications in CBSE/ICSE curricula. If a numbered-type system is being used, the likely mapping is:

• Type 1 = Natural
• Type 2 = Synthetic
• Type 3 = Universal
• Type 4 = Olfactory (smell-based, e.g., onion or vanilla change odour in acid/base)