If the size of a dimension of a cube is more than the normal (more than 20%) of the fact table, you define that dimension as a ine item dimension; for example if you store sales document number in one dimension in a sales cube, usually the dimension size and the fact table size will be the same; when you add the overhead of look ups for DIMID/SIDs the performance will be very slow; by lagging it as a ine item dimension, the system puts the SID in the fact table instead of DIMD for the sales document number; this avoids one look up into dimension table (the dimension table is not created in this case)
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