Using logic such as [Left String (bytes)] and [After Specified Character], you can specify bytes from the middle of a character string and obtain the expected data.
Sample data
xxxxxxxxxxxxIDDATAxx
IDDATA
Please refer to the following DataSpider Servista Help for details about each logic:
https://www.hulft.com/help/en-us/DataSpider/dss42sp2/help/en/mapper/document/logic/leftByte.html
https://www.hulft.com/help/en-us/DataSpider/dss42sp2/help/en/mapper/document/logic/substringAfter.html