Flagging a dimension as line item dimension changes the data modell by that it removes the intermediate SID table that is normally put between fact tables and the actual dimension tables (for flexibility reasons) and joins the fact table directly with the dimension table.

It changes this :

[DIMENSION-TAB1] >-----<  (SID-TAB1) >---< [[FACT TABLE]] >---<  (SID-TAB2) >-----< [DIMENSION-TAB2]

to this:

 [DIMENSION-TAB1] >-----< [[FACT TABLE]]  >---<  (SID-TAB2) >-----< [DIMENSION-TAB2]

References:

• http://scn.sap.com/community/data-warehousing/netweaver-bw/blog/2011/04/16/short-note-on-line-item-and-high-cardinality-dimensions
• http://help.sap.com/saphelp_nw70ehp1/helpdata/en/a7/d50f395fc8cb7fe10000000a11402f/frameset.htm
• BITMAP vs B*TREE (Oracle forum)
• BITMAP vs B*TREE (Stack Overflow)