Earned revenue is the revenue you have earned by doing the work, regardless of what you have billed. Under the cost-to-cost method it is the contract value multiplied by percent complete. A $12M contract that is 40% complete has earned $4.8M of revenue.
The contract value should include approved change orders. Changes still in negotiation are trickier: how and when pending or unapproved changes enter the contract value depends on your accounting policy and on how likely they are to be collected, so agree the treatment with your CPA and apply it consistently. The same caution applies to claims.
Earned revenue minus cost to date is the gross profit recognized to date. Because it follows the forecast, a change in estimated total cost changes earned revenue for the whole job to date, not just this month. That catch-up adjustment is why a bad estimate update can swing a month's income statement sharply.
If the projected job is a loss, the standard treatment is to recognize the entire expected loss as soon as it is known, rather than spreading it across the remaining work.