adverbials可以看作是 ADJUNCTS and SUBJUNCTS 的同位语，相当于 they are adverbials which are...。即：

Such sentence adverbials are distinguished from ADJUNCTS and SUBJUNCTS, (they are) adverbials which are more closely integrated with the rest of the clause, and which include such familiar categories as adverbials of manner, place, time, and degree.

其实去掉 adverbials 也完全可以（逗号后关系代词which指代ADJUNCTS and SUBJUNCTS）：

Such sentence adverbials are distinguished from ADJUNCTS and SUBJUNCTS, which are more closely integrated with the rest of the clause, and which include such familiar categories as adverbials of manner, place, time, and degree.

大意：此类句子状语与附加状语（ADJUNCTS）和下加状语（SUBJUNCTS）不同。它们（附加状语和下加状语）与从句的其他部分结合得更紧密，它们包括诸如方式、地点、时间和程度等。