Quote from Carsten's document:

The logical key of the PARTNER set includes the partner function and the partner itself, i.e., there is no external attribute which allows to differentiate all the records. For this case, we use the term internal key.
However in the system the fields Partner function and partner number are put to CRMT_PARTNER_EXT(Externally Influenced Partner Data), which makes me a little bit confused(囧).

Answer

CRMT_PARTNER_EXT has nothing to do with the distinction into internal and external key. In fact, it took me some time to decide for the words of these attributes. External means: There is some criterion outside the order which structures the records of the complex set. Internal means: You cannot tell from outside how many records of this entity will be present in the order, it can be an arbitrary number.

I am sorry I still have doubt for your clarification. I am sorry for that.

"External means: There is some criterion outside the order which structures the records of the complex set.

Internal means: You cannot tell from outside how many records of this entity will be present in the order, it can be an arbitrary number."

I still use DATES ( Appointment ) for example.
Below is a screenshot for variable lt_app.

DATA: lt_guid TYPE crmt_object_guid_tab,
          lt_app  TYPE crmt_appointment_wrkt.

    APPEND iv_guid TO lt_guid.

    CALL FUNCTION 'CRM_ORDER_READ'
      EXPORTING
        it_header_guid = lt_guid
      IMPORTING
        et_appointment = lt_app.

If I understand you correctly, since APPT_TYPE and DURATION type are external key, so outside the order we can exactly know that it would NEVER happen that there are two entries for an order which have the same APPT_TYPE and DURATION. Do I understand correctly?

Now for Partner,

you mentioned "The logical key of the PARTNER set includes the partner function and the partner itself",

From technical point of view, I don't see any difference compared with Appointment's logical key.

So I get the following assumption:

(1) There is NO technical difference between internal logical key and external logical key. The categorization is made from business point of view.
(2) Could I try to describe how to differentiate these two kinds of logical key below?
Say a component has logical key A and B.
The picture below shows the feature of external key:

The picture below shows the feature of internal key:

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