What do these expressions mean in general: first-level, second-level, … ?
For example,
Response rates provide a first-level indicator of advertisement
effectiveness. Conversion rates offer a second-level indicator of
advertisement effectiveness.
Learn English – Meaning of “first-level”, “second-level” in general
adjectivesmeaning
Best Answer
As comments have indicated, this is a specialised use and "first-level", "second-level" etc mean different things in different contexts. In this particular case, second-level is a better indicator than first-level.
If an advert is not effective, no-one will even respond to get more details of the product, let alone buy it.
Counting the number of responses to the advert will provide a measure of effectiveness. Counting the number of conversions — the number of actual purchases — will provide a better measure of effectiveness.
A more effective advertisement will have a higher number of conversions than a less effective one, although both might have around the same number of enquiry responses.
Because the quotation is ambiguous, I would phrase it