My guess - it is describing the effect on the Attack bar.
However, it also seems plausible that the intent is to always place one corrode plus a chance for an addtional corrode on 9+. In Arena this would have been shown as
1 - 8 Corrode
9+ 2 Corrode
This text might be the equivalent of this probability. Are there examples in Academy where a mutli-lined effect result is used?
The promo version of the card looked as above: e.g. 1 Corrode on 1-8, 2 Corrodes on 9+. However, this would mean that attacking an object with Acid -X (e.g. having Chitin Armor, for example) would provide a possibility of receiving 0 Corrodes. The officially released Academy card avoids this possibility by guaranteeing at least 1 Corrode, assuming the attack isn't avoided entirely.
In other words, the released version is almost functionally equivalent to the promo version, but actually better against targets with Acid -X.
You can still see the promo version here:
http://www.spellbookbuilder.com/