The Receipt Emails feature lets you set up rules for attaching emailed receipts for services you pay for, such as web hosting, to the corresponding explanations in FreeAgent.
Most people (including me!) use this for services billed monthly. Each time a new receipt email arrives, DoubleAgent looks for the next matching explanation without an attachment and uploads the receipt to it.
For example, in my rule for GitHub's receipts, I look for emails with a subject including the text "[GitHub] Payment Receipt" and bank explanations with a description including the text "GitHub".
This works perfectly because there's never any doubt about which explanation a receipt should be attached to.
But what if you're receiving several receipt emails a day – perhaps they are invoices for online sales you make – and you can't be sure the emails will arrive in the same order as the bank transactions? You could end up with receipts/invoices attached to the wrong explanations.
In your attachment rule, in the part where you specify which transaction explanations to look for, you can now get it to pull text from each individual email and look for that. (It's easier than I've made it sound.)
For example, let's say your receipt emails are order confirmations containing an order number; and that each sale's bank transaction contains the order number. You can have a rule which extracts the order number from the email and matches the specific transaction explanation with that order number.
This is the same extract-and-match technology which explainer rule descriptions have had for several years. Once you have wrapped your head around it, it can be super useful.
You can read more in the docs.