each getData() function seems to be calling pack(). Example of NDRSTRUCT

class NDRSTRUCT(NDRCONSTRUCTEDTYPE):
    def getData(self, soFar = 0):
        data = b''
        arrayPadding = b''
        soFar0 = soFar
        # 14.3.7.1 Structures Containing a Conformant Array
        # A structure can contain a conformant array only as its last member.
        # In the NDR representation of a structure that contains a conformant array, 
        # the unsigned long integers that give maximum element counts for dimensions of the array 
        # are moved to the beginning of the structure, and the array elements appear in place at 
        # the end of the structure.
        # 14.3.7.2 Structures Containing a Conformant and Varying Array
        # A structure can contain a conformant and varying array only as its last member.
        # In the NDR representation of a structure that contains a conformant and varying array, 
        # the maximum counts for dimensions of the array are moved to the beginning of the structure, 
        # but the offsets and actual counts remain in place at the end of the structure, 
        # immediately preceding the array elements
        lastItem = (self.commonHdr+self.structure)[-1][0]
        if isinstance(self.fields[lastItem], NDRUniConformantArray) or isinstance(self.fields[lastItem], NDRUniConformantVaryingArray):
            # So we have an array, first item in the structure must be the array size, although we
            # will need to build it later.
            if self._isNDR64:
                arrayItemSize = 8
                arrayPackStr = '<Q'
            else:
                arrayItemSize = 4
                arrayPackStr = '<L'

            # The size information is itself aligned according to the alignment rules for 
            # primitive data types. (See Section 14.2.2 on page 620.) The data of the constructed 
            # type is then aligned according to the alignment rules for the constructed type. 
            # In other words, the size information precedes the structure and is aligned 
            # independently of the structure alignment.
            # We need to check whether we need padding or not
            pad0 = (arrayItemSize - (soFar % arrayItemSize)) % arrayItemSize 
            if pad0 > 0:
                soFar += pad0
                arrayPadding = b'\\xee'*pad0
            else:
                arrayPadding = b''
            # And now, let's pretend we put the item in
            soFar += arrayItemSize
        else:
            arrayItemSize = 0

        # Now we need to align the structure 
        # The alignment of a structure in the octet stream is the largest of the alignments of the fields it
        # contains. These fields may also be constructed types. The same alignment rules apply 
        # recursively to nested constructed types.
        alignment = self.getAlignment()

        if alignment > 0:
            pad = (alignment - (soFar % alignment)) % alignment
            if pad > 0:
                soFar += pad
                data += b'\\xAB'*pad

        for fieldName, fieldTypeOrClass in self.commonHdr+self.structure:
            try:
                if isinstance(self.fields[fieldName], NDRUniConformantArray) or isinstance(self.fields[fieldName], NDRUniConformantVaryingArray):
                    res = self.fields[fieldName].getData(soFar)
                    if isinstance(self, NDRPOINTER):
                        pointerData = data[:arrayItemSize]
                        data = data[arrayItemSize:]
                        data = pointerData + arrayPadding + pack(arrayPackStr ,self.getArrayMaximumSize(fieldName)) + data
                    else:
                        data = arrayPadding + pack(arrayPackStr, self.getArrayMaximumSize(fieldName)) + data
                    arrayPadding = b''
                    arrayItemSize = 0
                else:
                    res = self.pack(fieldName, fieldTypeOrClass, soFar)
                data += res
                soFar = soFar0 + len(data) + len(arrayPadding) + arrayItemSize

if the base type for this class is NDR ,then the pack will point to Ndr.pack()

content of dcerpc\\v5\\ndr.py

def pack(self, fieldName, fieldTypeOrClass, soFar = 0):
        if isinstance(self.fields[fieldName], NDR):
            return self.fields[fieldName].getData(soFar)
## - 
        data = self.fields[fieldName]
        # void specifier
        if fieldTypeOrClass[:1] == '_':
            return b''
## = 
        # code specifier
        two = fieldTypeOrClass.split('=')
        if len(two) >= 2:
            try:
                return self.pack(fieldName, two[0], soFar)
            except:
                self.fields[fieldName] = eval(two[1], {}, self.fields)
                return self.pack(fieldName, two[0], soFar)

        if data is None:
            raise Exception('Trying to pack None')
## ：
        # literal specifier
        if fieldTypeOrClass[:1] == ':':
            if hasattr(data, 'getData'):
                return data.getData()
            return data

        # struct like specifier
        return pack(fieldTypeOrClass, data)

Noted the above specifier is what’s shown in the \\impacket\\[structure.py](<http://structure.py/>)