Fold tag represents a listing of items, which are added to the project. If you make an analogy with GUI, it contains the following structure:
It consists of <Item>
tags, sepearted by other unknown fields, possibly showing beginning/end, or position in the list, or no idea what.
<Fold>
<fdta bdata="0000000000000000000000000000"/>
<Item>
<idta bdata="000700000000000000000000000000000000000c000000300000000000000000000000000000000000000000000000000000000000000000000005000000000000000000000000000000000000000000d655bb34"/>
<string/>
<Pin>
<sspc bdata="000000000000000000000000000000000000000000005a5045470000000000000500000002d00000000000000258000000000000025800000000000000000018010100000000000000030000000000010000000000000000000000000000000000000000000000000000000000000000002000000000d655bb340000000c0000000100000000000000000001000000010000000000000000000000000000000800000000000000000000000000000000000000000000000000000000ffffffffffffffff0000000100020000000000000009f20b01000000000000000000"/>
<string/>
<Als2>
<fileReference ascendcount_base="1" ascendcount_target="3" fullpath="D:\Projects\ae-test\nexrender-boilerplate-master\assets\2016-aug-deep.jpg" platform="Win" server_name="inlife-pc" server_volume_name="HDD" target_is_folder="0"/>
</Als2>
<opti bdata="5a50454700050000003a00000000000000000000000000000000000000004745504affffffff5241572001000000010000000000000000000000"/>
<pgui bdata="974ff14f0434432e970b4e3f09223407"/>
<CLRS>
<epid bdata="ffffffffffffffffffffffffffffffff"/>
<apid bdata="601e997329e4901f2acc5b497ba315f3"/>
<linl bdata="00000000"/>
<embp bdata="01"/>
<ipws bdata="00"/>
</CLRS>
<mnfo>
<strt bdata="000000001e000000"/>
<drop bdata="01"/>
</mnfo>
<string/>
</Pin>
<ftgi bdata="0000000000000001ffffffff00000258"/>
<string/>
<Gide>
<gdta bdata="0000000000000000"/>
<list>
<lhd3 bdata="00d00bee000000000000000000000001000000100000000200000001000000020000000000000000000000000000000000000000"/>
</list>
</Gide>
</Item>
<fvdv bdata="00000003"/>
<fiop bdata="00"/>
<ftts bdata="00000000"/>
<foac bdata="00"/>
<fiac bdata="00"/>
<fipc bdata="0000"/>
<fifl bdata="00000000"/>
<Item>
...
</Fold>
Not empty Fold will always contain at least one Compossition item (depends on how many compositions you've added to the project), more details: Composition.
Composition takes the most space for xml definition.
Other Items are described there: Item