I've heard that there are F13 - F24 function key by holding Shift while press F1-F12.

What do thoses extra function key do?

Hi virgosun , 

Greetings. 

On older legacy /multifunctional/ apple keyboards such keys where physically assigned to perform needed function at that time. 

ISO/IEC 9995 Keyboard layouts for text and office systems is an ISO standard series defining layout principles for computer keyboards. It does not define specific layouts but provides the base for national and industry standards which define such layouts. 

History :

• IBM 5250: early models frequently had a "cmd" modifier key, by which the numeric row keys emulate function keys; later models have either 12 function keys in groups of 4 (with shifted keys acting as F13–F24), or 24 in two rows. These keys, along with "Enter," "Help," and several others, generate "AID codes," informing the host computer that user-entered data is ready to be read.
• IBM PC AT and PS/2 keyboard: F1 to F12 usually in three 4-key groups across the top of the keyboard. The original IBM PC and PC XT keyboards had function keys F1 through F10, in two adjacent vertical columns on the left hand side; F1|F2, F3|F4, ..., F9|F10, ascending downwards. Some IBM compatible keyboards, e.g., the NorthgateOmniKey™/102, also featured function keys on the left, which on examples with swapped left Alt and Caps Lock keys, facilitate fingers of a single hand simultaneously striking modifier key(s) and function keys swiftly and comfortably by touch even by those with small hands. Many modern PC keyboards also include specialized keys for multimedia and operating system functions.

Functions based on scan codes :

17     43   43   0C  I         E0 47* E0 6C*  6E   4A  Home CP
18     44   44   12  O            48     75   75   60  8 Up KP        5B     1F   08   68  F13
19     4D   4D   13  P         E0 48* E0 75*  63   52  Up CP          5C     27   10   69  F14
1A     54   54   2F  { [          49     7D   7D   61  9 PgUp KP      5D     2F   18   6A  F15
1B     5B   5B   30  } ]       E0 49* E0 7D*  6F   4B  PgUp CP        63     5E   2C   6B  F16
1C     5A   5A   28  Enter        4A     7B   84   56  - KP           64     08   2B   6C  F17
E0 1C  E0 5A   79   58  Enter KP     4B     6B   6B   5C  4 Left KP      65     10   30   6D  F18
1D     14   11   E0  Ctrl L    E0 4B* E0 6B*  61   50  Left CP        66     18   38   6E  F19
E0 1D  E0 14   58   E4  Ctrl R       4C     73   73   97  5 KP           67     20   40   6F  F20
1E     1C   1C   04  A            4D     74   74   5E  6 Right KP     68     28   48   70  F21     
1F     1B   1B   16  S         E0 4D* E0 74*  6A   4F  Right CP       69     30   50   71  F22
20     23   23   07  D            4E     79   7C   57  + KP           6A     38   57   72  F23
21     2B   2B   09  F            4F     69   69   59  1 End KP       6B     40   5F   73  F24

Such features may just be another unknown keys different from how each generations of windows would need it.

Other softwares may also offer better output result instead.

Hope this help answer your query.

Regards