Often times, particularly in enterprise software, part of the terms of a deal include a "source code escrow" clause. This means that if you go out of business, the customer with this term gets your platform's source code released to them so their investment into your technology doesn't implode completely.

@kevinpfab is right, but it can include other clauses besides just if you go out of business. For instance, a sale of your company could be an escrow worthy problem because the new company may stop development, increase prices, offer a lower SLA, etc