In order to use a conventional expression, you must have a copy physically present with you. Even television and radio programs must be transmitted to your physical receiver, where with the right equipment you can make a copy of the expression. With dynamic expressions, this is not true. A program can live on a foreign server and it may be impossible to get a copy for yourself.
What does this mean? It is impossible to archive a copy of these expressions. It's impossible to copy these expressions at all, for instance to make "fair use" of them, or to use one's "first sale" privileges.
The entire "ASP" (Application Service Provider) buzz of 2000-2001 was built around this idea that companies can host applications on servers that anybody can use from anywhere (or so the theory goes). It may be convenient, but it also means that if some company decides to purchase service from some ASP, it is technically impossible for them to obtain an archive copy of the "Service" (which consists of software) without the agreement of the ASP. We have the legal right to archive certain types of content, but in order for us to archive something, there has to be something local to archive! Much of the drive around ASP's on the business end derived from the impossibility of pirating the applications (or indeed even canceling your subscription in some cases, if the data was held hostage on the ASP servers, rather then the user's local hard drive), and allowing the ASP to fully control the use of the application.
The "ASP" label died with the dot-com crash, but the concept still lives on in almost every dynamic web page on the Internet. For instance, try archiving the software Microsoft uses to run Hotmail. You can't even access that software expression. You can only see the results of the software expression's execution as the Hotmail web pages.
In terms of derivation trees, it is as if that top link to the consumer has been severed. All of the links in the past required the physical presence of a copy, which implied the ability to do certain things, like make more copies of it. Some of these abilities were codified into what is now called the "First Sale Doctrine". If you do not have a local copy, suddenly those 'rights' become meaningless, because it is impossible to physically perform the acts necessary to copy something. This is having the very real effect of causing people to question the First Sale Doctrine, and some people with economic interests in not having the First Sale Doctrine around are trying to take advantage of this questioning to assert that the First Sale Doctrine should be eliminated entirely.
Note that a normal static web page is not non-local, as the web page itself is downloaded to your computer, and you can make a copy of that. For a dynamic web page, the static web page you receive is local, but the instructions on how to create that web page remain non-local, residing only as software on the original web server. This leads us quite naturally to the idea of...