every object you add to your potion belt window is considered a "piece" and will need to be added to the pieces list. so if you are adding the player hp bar gauge, you would need to add a line to the pieces list for it. make sure the names are exactly identical... i'll give you an example, this is the hp gauge "piece" from the default player window:
<Gauge item="Player_HP">
<ScreenID>PlayerHP</ScreenID>
<!--<Font>3</Font>-->
<TextColor>
<R>240</R>
<G>240</G>
<B>240</B>
</TextColor>
<RelativePosition>true</RelativePosition>
<Location>
<X>4</X>
<Y>3</Y>
</Location>
<Size>
<CX>108</CX>
<CY>41</CY>
</Size>
<TextOffsetX>8</TextOffsetX>
<GaugeOffsetY>31</GaugeOffsetY>
<Style_VScroll>false</Style_VScroll>
<Style_HScroll>false</Style_HScroll>
<Style_Transparent>false</Style_Transparent>
<!--<TooltipReference/>-->
<FillTint>
<R>240</R>
<G>0</G>
<B>0</B>
</FillTint>
<LinesFillTint>
<R>220</R>
<G>220</G>
<B>0</B>
</LinesFillTint>
<DrawLinesFill>false</DrawLinesFill>
<EQType>1</EQType>
<GaugeDrawTemplate>
<Background>A_GaugeBackground</Background>
<Fill>A_GaugeFill</Fill>
<Lines>A_GaugeLines</Lines>
<LinesFill>A_GaugeLinesFill</LinesFill>
<EndCapLeft>A_GaugeEndCapLeft</EndCapLeft>
<EndCapRight>A_GaugeEndCapRight</EndCapRight>
</GaugeDrawTemplate>
</Gauge>
if you are going to add this to the potion belt, i would recommend changing its name and screen ID to something like this:
<Gauge item="PB_Player_HP">
<ScreenID>PB_PlayerHP</ScreenID>
sometimes the UI gets confused if more than one xml has a piece with the same names in them, and will error into loading the default UI. why i don't know /shrug
then at the bottom, where the pieces list is in your potion belt xml, you would add a line that says this:
<Pieces>PB_Player_HP</Pieces>
the pieces line needs to have the exact same name as you gave the gauge in the potion belt window.
hope that helps you out =)