In Weapons of Mass Destruction and Terrorism by James Forest and Russel Howards, the second edition, what does it say are some key generalizations about critical infrastructure that are especially relevant in the context of terrorism?

There are essentially three key generalizations about critical infrastructure (CI) that are especially relevant in the context of terrorism:
1) The definition of CI varies because it spans a vast and diverse set of industries, traditions, assets, and people.
For example, the definition of CI will vary depending on location. Large cities may count power reactors as CI, but there may not be corresponding CI in rural areas. Also, it is a challenge to classify CI into distinct categories. This is because similar systems can have different component parts, depending on location. Take for example, the classification of the government facilities sector.
In large cities, such CI may comprise national laboratories and military installations that handle information and assets crucial to national security. There may not be corresponding CI in rural areas. Although there may be small military installations in rural areas, they will not host a corresponding level of national security assets a large installation in a major city will.
2) Not all CI are similarly vital.
According to the book definition, CI comprises the pertinent systems and services essential to the functioning of everyday life. Here, such a definition will vary, according to location. What is considered essential to "everyday life" will vary, depending upon local or national considerations.
For example, Al-Qaida or Islamic State terrorists are more likely to attack major CI in large cities because such attacks cause maximum psychological impact. Take for example, the Berlin Christmas attack in 2016. Also, the 2001 Twin Towers attack. The Twin Towers was considered a major cultural, political, and economic institution. Terrorists often target CI of cultural or social significance to their intended victims. Again, what is considered "essential" in rural life will differ from what is considered "essential" in city culture.
3) CI can now be classified as "physical" or "cyber" assets.
To date, cyber CI are increasingly critical, and it is crucial to recognize threats to them. For example, between 2010 and 2014, the United States Department of Energy suffered more than 150 cyber breaches. Now, there are new cybersecurity threats to the transportation industry. So, it is imperative to recognize the differences between "physical" and "cyber" assets so that counter-terrorism officials can formulate specific plans to protect both types of CI.