When creating a sales or purchase document, you have the option to link it with a project. Additionally, you can add a project to each product or service line of the document. This helps to combine potential jobs related to different projects for one customer into one document. Plus, in each project's detailed view you can see the data of that specific job.
In addition, it's a good way to split costs between different projects, when using this option on a bill.
Let's take a look at how it works, for example, with quotes.
When creating a quote, add all relevant data like you're used to, and when selecting to link the quote to a project, pick the option Project on each line.
Then, you can select a different project for each product or service line on the document.