Learn Russian: Пассажиры должны соблюдать правила досмотра, за исключением послов. - Passengers must observe screening rules, with the exception of ambassadors.

The compound preposition за исключением (with the exception of) always requires the noun that follows it to be in the Genitive case. Послов is the Genitive plural of посол (ambassador).

The dictionary form is посол. Where did the second 'о' go in послов?

Посол has a fleeting vowel in its final syllable. When case endings are added, this 'о' drops out. This gives us the stem посл-, which takes the standard masculine Genitive plural ending -ов to become послов.

Why is правила досмотра used instead of an adjective to say 'screening rules'?

Russian frequently uses a noun in the Genitive case to modify another noun, where English just stacks nouns together. Правила досмотра literally translates to 'rules of screening.' This Noun + Genitive Noun structure is the standard way to form official terms and compound concepts.

Why does должны take the imperfective verb соблюдать here?

The imperfective is used because observing rules is an ongoing, continuous requirement, not a one-time completed action. Whenever someone is expected to follow rules or laws continuously, you will see the imperfective соблюдать.

Is the comma before за исключением mandatory?