As the name suggests, between every black bar (expressing former character), white bars (expressing latter character) are inserted. Then the combination express 2-digit number with five black bars and five white bars.
It appears as if five bars express 2-digit code, and therefore has higher density than other 2 of 5 barcodes.
Summary of Interleaved 2 of 5 Symbol
|Length of symbol||variable, but must be even length
(fixed to 6/14/16 digits in case of UPC Case Code, Standard Symbology for Physical Distribution)
(uses modulus 10 in case of UPC Case Code, Standard Symbology for Physical Distribution)
|Character Density||about 7.1 chars/cm
(about 5 to 1 chars/cm in case of Standard Symbology for Physical Distribution)