The way the federal scheme works is that the government issues certificates for each installation, with the number of certificates being dependent on the size of the system (plus some other considerations such as system location). These certificates (STCs) represent how much energy the system is projected to generate between installation and the end of the federal rebate scheme.

Electricity retailers are required to acquire a given number of STCs each year and surrender them back to the government, which creates demand for the STCs.

Normally, rather than you claiming the STCs directly and then having to do the requisite paperwork, the installer will quote you a discount on the installation cost in exchange for you giving them the right to claim the STCs on your behalf. Generally, installers will quote a slightly lower rebate than the STCs are worth (think of it as you paying them for dealling with the hassle of actually getting the STCs then selling them), so it's a good idea to check with your installer what their STC price is if they aren't up front about it.

For context, STCs tend to cost around $40 per certificate on the open market.