Attributes can be related to or are income sources. Income sources are covered thoroughly in subsequent pages. However in attributes, you are interested only in key income sources. The reason for having key income sources as attributes is that they help you to formulate the business idea in the initial stages.
An example of such an attribute could be advertisements. For instance, if your business idea is a blog, you may want to get income from both product sales and advertisements. Similarly attributes can include giving free access to customers but charging service providers which want to be listed on your website. Thus advertisements is a key income source as well as an attribute for your business idea.
Another example of an attribute which is a key income source is sale of supporting products/services. For instance let’s say your business idea is to ride on an existing software which it is given away free. There is a wealth of open source software available for many uses at no charge. There is thus no revenue to be earned from the sale of such software. However you could offer, say, a training program, to teach users on the use of open source software. Alternatively you could provide supporting services for users.