What is output data of bar code reader?

Typically just the data that the reader encountered. If it's a UPC code the scanner would output the data on the barcode.

Most scanners can also be programmed to include some kind of preamble data, which may simply be a beginning of text marker or an identifier from the scanner.

Scanners can also send "post" data, such as a <CR> or some other kind of identifier that indicates that the scanner has completed a good scan.

Scanners can be programmed to ignore some types of barcode and not others.

It all depends on the needs of the individual application.