A quick Trace Parse will show this. Below is the query it runs when the On-Hand is called.
INVENTSUM is where your inventory values are stored but INVENTTRANS plays a part as well which is where you need to be careful as far as pitfalls go. It's not enough to just SUM the inventory but SUM (or subtract) against the right status types, i.e., AVAILPHYSICAL, REGISTERED, ON ORDER, etc.
---
SELECT SUM(T1.POSTEDQTY),
SUM(T1.POSTEDVALUE),
SUM(T1.PHYSICALVALUE),
SUM(T1.DEDUCTED),
SUM(T1.REGISTERED),
SUM(T1.RECEIVED),
SUM(T1.PICKED),
SUM(T1.RESERVPHYSICAL),
SUM(T1.RESERVORDERED),
SUM(T1.ONORDER),
SUM(T1.ORDERED),
SUM(T1.ARRIVED),
SUM(T1.QUOTATIONRECEIPT),
SUM(T1.QUOTATIONISSUE),
SUM(T1.PHYSICALINVENT),
SUM(T1.AVAILPHYSICAL),
SUM(T1.AVAILORDERED),
SUM(T1.PDSCWPOSTEDQTY),
SUM(T1.PDSCWDEDUCTED),
SUM(T1.PDSCWREGISTERED),
SUM(T1.PDSCWRECEIVED),
SUM(T1.PDSCWPICKED),
SUM(T1.PDSCWRESERVPHYSICAL),
SUM(T1.PDSCWRESERVORDERED),
SUM(T1.PDSCWONORDER),
SUM(T1.PDSCWORDERED),
SUM(T1.PDSCWARRIVED),
SUM(T1.PDSCWQUOTATIONRECEIPT),
SUM(T1.PDSCWQUOTATIONISSUE),
SUM(T1.PDSCWPHYSICALINVENT),
SUM(T1.PDSCWAVAILPHYSICAL),
SUM(T1.PDSCWAVAILORDERED),
T1.ITEMID,
T2.INVENTLOCATIONID,
T2.INVENTBATCHID,
T2.WMSLOCATIONID,
T2.WMSPALLETID,
T2.INVENTSERIALID,
T3.NAMEALIAS
FROM INVENTSUM T1 CROSS
JOIN INVENTDIM T2 CROSS
JOIN INVENTTABLE T3
WHERE (((T1.PARTITION=?)
AND (T1.DATAAREAID=?))
AND (T1.CLOSED=?))
AND (((T2.PARTITION=?)
AND (T2.DATAAREAID=?))
AND (T1.INVENTDIMID=T2.INVENTDIMID))
AND (((T3.PARTITION=?)
AND (T3.DATAAREAID=?))
AND (T1.ITEMID=T3.ITEMID))
GROUP BY T1.ITEMID,
T2.INVENTLOCATIONID,
T2.INVENTBATCHID,
T2.WMSLOCATIONID,
T2.WMSPALLETID,
T2.INVENTSERIALID,
T3.NAMEALIAS
ORDER BY T1.ITEMID OPTION(FAST 14)
