consider a trade company which consists of fact about trading that take places in a share trading firm.in these there may be some facts like mode of trade(which indicates whether the user is trading through phone or online)which will be not related to any of the dimensions such as account,date,indices,amount of share etc.
so these unrelated facts are removed from the fact table and stored as a separate dimension as a junk dimension which will be useful to provide extra information
A junk dimension is a table containing all known or given
combinations of two or more other dimensions. It is then
used as a lookup from a fact table to find the combination
of dimensions that relate to that fact. The purpose is to
declutter and reduce the width of the fact table.
An example might be a [SalesFact] table which references a
junk dimension [SalesExtentionDIM] containing values/SKs
for dimensions of [OnSaleYN] , [SaleCodeSK] , [ColourSK] ,