Typically, the technical and business people at the client have decided that they need some new product to help them in their business, and they create and send out an RFP describing what they're looking to buy.
Proposals typically contain detailed claims about capabilities and so on. After proposal reviews (and maybe presentations), the customer will select a vendor. At this point, the client's lawyer gets called in: usually after the RFP and the presentations. That's a shame, of course, but it's typical.
So you get the license agreement from the proposer, and your client says "can we sign this?" Begin the dance.
Sometimes the purchase will involve both hardware and software. This will be the software side of the deal, though, and that's generally what the client wants to focus on: what can it do for their business.
She differentiates between "system software" and "application software." Software is usually licensed, not sold (so the owner retains whatever trade secrets there are). Some licenses are exclusive, others non-exclusive.