LINKAGE SECTION: It is used to access the data that are external to the program. JCL can send maximum 100 characters to a program thru PARM. Linkage section MUST be coded with a half word binary field, prior to actual field. If length field is not coded, the first two bytes of the field coded in the linkage section will be filled with length and so there are chances of 2 bytes data truncation in the actual field.
The linkage section is part of a called program that 'links' or maps to data items in the calling program's working storage. It is the part of the called program where these share items are defined.