And this indicator does this over 5 different forex brokers.
The common definition for real-time relative volume is: Current volume for that volume bar / Average volume bar on that moment. It does mean taking the volume (for example) from 10:30 am to 10:45 am and comparing it to what it does from 10:30 am to 10:45 am every day.
In true TradingView spirit, the author of this script has published it open-source, so traders can understand and verify it. Cheers to the author! You may use it for free, but reuse of this code in a publication is governed by House Rules. You can favorite it to use it on a chart.