The use of /D (number) inside /A action on PDF specification language

Question

what is the purpose of /D (a number) inside /A action dictionary inside an annotation dictionary, on PDF specification language, how is its mechanism works
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218 0 obj
<< /Type /ObjStm /Length 8647 /N 100 /First 889 >>
stream
219 0 220 116 221 237 222 353 223 470 224 587 225 704 226 820 227 936 228 1053
<< /A << /D (116) /S /GoTo >> /Border [ 0 0 0 ] /Rect [ 433.636 624.06 450 631.636 ] /Subtype /Link /Type /Annot >>
<< /A << /D (117) /S /GoTo >> /Border [ 0 0 0 ] /Rect [ 426.336 591.183 450.545 603.935 ]     /Subtype /Link /Type /Annot >>
<< /A << /D (117) /S /GoTo >> /Border [ 0 0 0 ] /Rect [ 433.636 575.413 450 585.11 ] /Subtype /Link /Type /Annot >>
<< /A << /D (117) /S /GoTo >> /Border [ 0 0 0 ] /Rect [ 433.636 564.384 450 571.959 ] /Subtype /Link /Type /Annot >>
<< /A << /D (118) /S /GoTo >> /Border [ 0 0 0 ] /Rect [ 433.636 551.233 450 558.809 ] /Subtype /Link /Type /Annot >>
<< /A << /D (118) /S /GoTo >> /Border [ 0 0 0 ] /Rect [ 433.636 538.082 450 545.658 ] /Subtype /Link /Type /Annot >>
<< /A << /D (119) /S /GoTo >> /Border [ 0 0 0 ] /Rect [ 433.636 522.81 450 532.507 ] /Subtype /Link /Type /Annot >>
<< /A << /D (119) /S /GoTo >> /Border [ 0 0 0 ] /Rect [ 433.636 509.66 450 519.357 ] /Subtype /Link /Type /Annot >>

...

while no /D ( ) explained in Action on PDF ISO32000-2 reference, i.e:

12.6.2 Action dictionaries An action dictionary defines the characteristics and behaviour of an action. "Table 196: Entries common to all action dictionaries" shows the required and optional entries that are common to all action dictionaries. The dictionary may contain additional entries specific to a particular action type; see the descriptions of individual action types in 12.6.4, "Action types" for details.

Table 196: Entries common to all action dictionaries

Key Type Value

Type name (Optional) The type of PDF object that this dictionary
describes; if present, shall be Action for an action dictionary.
S name (Required) The type of action that this dictionary describes; see "Table 201: Action types" for specific values.
Next dictionary/array (Optional; PDF 1.2) The next action or sequence of
actions that shall be performed after the action represented by this dictionary. The value is either a single action dictionary or an array of action dictionaries that shall be performed in order; see Note 1 for further discussion.

NOTE 1 The action dictionary’s Next entry (PDF 1.2) allows sequences of actions to be chained together. For example, the effect of clicking a link annotation with the mouse might be to play a sound, jump to a new page, and start up a movie. Note that the Next entry is not restricted to a single action but may contain an array of actions, each of which in turn may have a Next entry of its own. The actions may thus form a tree instead of a simple linked list. Actions within each Next array are executed in order, each followed in turn by any actions specified in its Next entry, and so on recursively. It is recommended that interactive PDF processors attempt to provide reasonable behaviour in anomalous situations. For example, self-referential actions ought not be executed more than once, and actions that close the document or otherwise render the next action impossible ought to terminate the execution sequence. Applications need also provide some mechanism for the user to interrupt and manually terminate a sequence of actions. PDF 1.5 introduces transition actions, which allow the control of drawing during a sequence of actions; see 12.6.4.15, "Transition actions". NOTE 2 It is recommended that no action modify its own action dictionary or any other in the action tree in which it resides. The effect of such modification on subsequent execution of actions in the tree is undefined.

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