Non-specific
Look at this sentence:

There were Germans on board the ship we took from Athens.

Here we are identifying the people by their nationality only. They are not specific. To be more specific we would have to say something like:

The Germans who were on the ship from Athens were very friendly.

This makes the reference specific.

Now look at these sentences:

The local car dealer has started to sell BMWs and Jaguars. People must be getting richer round here!

There is a good selection of Macs in that computer store.

In these sentences an unspecified number of products are referred to by their brand names only. Because brand names are used there is a degree of specificity, but they are still NON SPECIFIC because of the vagueness of numbers and styles.

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All inclusive
Look at these sentences:

Moslems do not eat pork.

Marxists are not taken very seriously anymore.

Buddhists believe that all existence is suffering.

If we refer to people by their religion, political beliefs, nationalities etc. we do not use an article if we mean that all members of a group are included in the statement.

We can also refer to brand names and days of the week in the same way:
e.g.
Ducatis are excellent motorcycles.

The garbage collectors come on Tuesdays.

However, if we refer to all the members of a family  we must use the article the:
e.g.
The Tanakas had to move house after the earthquake.

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If another determiner or possessive is used
If another determiner is used before a plural proper noun, the article the is not used:
e.g.
Most Irish are Catholics.

All Rolls Royces are expensive.

If a possessive pronoun is used or a possessive proper noun then no article is needed. Such sentences are quite rare.
e.g.
Hiroko's two Yorkshire Terriers are getting very old now.

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Directly preceded by a clause marker
Occasionally, especially in questions, a clause marker (i.e. which, what, whose) will directly precede a plural proper noun. In these cases no article is required:
e.g.

Which Mitsubishis were recalled by the company for repair?

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Before most numbers
If a number is used with a plural proper noun, then an article is usually unnecessary:
e.g.

Three Ferraris took part in the Le Mans 24-hour race.

Two Buddhists attended the theological conference in Kobe.

Of course, if there is PREVIOUS MENTION or SPECIFICITY involved then the article the must be used:
e.g.

Three Ferraris and two Jaguars took part in the Le Mans 24-hour race. The three Ferraris had been in the race before, but the two Jaguars were competing for the first time. (previous mention)

The two Buddhists who attended the theological conference in Kobe were from the Mahayana sect. (specificity)

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