Firms can supplement their income from the sale of supporting products and/or services. The amount that can be earned from such sales can be significant.
For instance, suppose you have a business distributing certain IT products, the predominent income comes from the sale of these main products. If the product manufacturer or main distributor does not provide any training on these products, herein lies an opportunity. You can source for someone knowledgeable on the use of these products to come up with a training program. The income generated is thus from the sale of a supporting service.
Alternatively, the software can be open source in which case you do not receive any income from it. However, lets say you assess that there is a demand from less knowledgeable users on the use of the open source software. Similar to the earlier example, you can offer such a training program targeted at these open source users.