What is the ERC (European Research Council)? Find more information here:

-ERC website
-Euresearch: the Swiss guide to European research & innovation

Q: What is the definition of "major equipment" for the additional budget request?

The ERC / ERCEA is probably deliberately unclear about this.

Equipment above 100 000 euro  might be a useful guideline.

The applicant has to convince the panel members reviewing the proposal that the additional funding is justified and required. The panel will then judge if it fits the description of major equipment (for their field/speciality) and if it is justified enough to be funded.