I have this code i need help explaining COU

here are my tables please explain to me the @QUANITYTOSELL VARIABLE USED AS I DON'T SEE HOW IT IS INITIALIZED INSTEAD IT IS JUST BEING USED

SELECT $*$ FROM TBLPRODUCTTRYCATCH

SELECT $*$ FROM TBLPRODUCTSALESTRYCATCH

$---$STORED PROCEDURE WHICH SALES PRODUCT AND IT WORKS WITH REAL TIME$------$

ALTER PROC SPSELLPRODUCT

@PRODUCTID INT,

@QUANTITYTOSELL INT

AS

BEGINDECLARE @STOCKAVAILABLE INTFROM TBLPRODUCTTRYCATCH WHERE PRODUCTID$=$@PRODUCTIDIF$($@STOCKAVAILABLE$$@QUANTITYTOSELL$)$ RAISERROR$(\text{'}$NOT ENOUGH STOCK AVAILABLE',$16,1)---$IF ENOUGH STOCK AVAILABLE$---$ BEGIN $---$FIRST REDUCE THE QUANTITY AVAILABLE$---$ WHERE PRODUCTID$=$@PRODUCTID $---$CALCULATE MAXPRODUCTSALESID MAX$($PRODUCTSALESID$)$ IS NULL FROM TBLPRODUCTSALESTRYCATCH SET @MAXPRODUCTSALESID$=$@MAXPRODUCTSALESID$+1$ IF$($@@ERROR ROLLBACK TRAN END BEGIN PRINT'TRAN COMMITTED' END $---$STORED PROCEDURE WHICH SALES PRODUCT AND IT WORKS WITH REAL TIME$------$

ALTER PROC SPSELLPRODUCT

@PRODUCTID INT,

@QUANTITYTOSELL INT

AS

BEGINDECLARE @STOCKAVAILABLE INTFROM TBLPRODUCTTRYCATCH WHERE PRODUCTID$=$@PRODUCTIDIF$($@STOCKAVAILABLE$$@QUANTITYTOSELL$)$ RAISERROR$(\text{'}$NOT ENOUGH STOCK AVAILABLE',$16,1)---$IF ENOUGH STOCK AVAILABLE$---$ BEGIN $---$FIRST REDUCE THE QUANTITY AVAILABLE$---$ WHERE PRODUCTID$=$@PRODUCTID $---$CALCULATE MAXPRODUCTSALESID MAX$($PRODUCTSALESID$)$ IS NULL FROM TBLPRODUCTSALESTRYCATCH SET @MAXPRODUCTSALESID$=$@MAXPRODUCTSALESID$+1$ IF$($@@ERROR ROLLBACK TRAN END BEGIN PRINT'TRAN COMMITTED' END