LE, what do those elements in red mean, what happens to the value?

<Data ElementName="offers_quest" Type="EQ2_8Bit_String" Size="1" IfVariableNotSet="footer_set_name_0" />
<Data ElementName="part_of_quest" Type="EQ2_8Bit_String" Size="1" IfVariableNotSet="footer_set_name_0" />
<Data ElementName="offers_quest_color" Type="int8" IfVariableSet="footer_offers_quest_0" />
<Data ElementName="part_of_quest_color" Type="int8" IfVariableSet="footer_part_of_quest_0" />
<Data ElementName="unknown_set_data1" Type="int8" Size="1" IfVariableSet="footer_set_name_0" />
<Data ElementName="charges" Type="int8" IfVariableNotSet="footer_set_name_0" />
<Data ElementName="total_charges" Type="int16" IfVariableSet="footer_charges_0" />
<Data ElementName="charges_left" Type="int16" IfVariableSet="footer_charges_0" />
<Data ElementName="unknown_set_info" Type="int32" Size="1" IfVariableSet="footer_set_name_0" />
<Data ElementName="unknown_set_info2" Type="int8" Size="1" IfVariableSet="footer_set_name_0" />
<Data ElementName="unknown_set_info3" Type="int32" Size="1" IfVariableSet="footer_unknown_set_info2_0" />

IfVariableNotSet="FIELD" means that if the FIELD is a 0, then this data element is used, otherwise it is skipped.

IfVariableSet="FIELD" means that if the FIELD is NOT 0, then this data element is used, otherwise it is skipped.

In your example if the footer_set_name_0 field for the packet is 0 then the offers_quest field is used, otherwise it is skipped.
Likewise, unknown_set_data1 is only used if footer_set_name_0 is set. Its just my way of trying to handle all the crazy curveballs that SOE throws into their data.

OK, just trying to wrap my brain around this stuff, lol....

So, if there is there is no value for set_name (meaning its = 00), then...
<Data ElementName="unknown_set_data1" Type="int8" Size="1" IfVariableSet="footer_set_name_0" /> is skipped and
<Data ElementName="offers_quest" Type="EQ2_8Bit_String" Size="1" IfVariableNotSet="footer_set_name_0" /> is used.

So being skipped I assume that there is no value there at all? (i.e. not a 00)

That is correct. Being skipped means that it will not be used when loading a data packet or creating a new data packet. To the client, that field never existed.