You can use the Data Element Separation method Use Data Pattern to support a variable number of repetitions in an otherwise fixed length environment. However, it relies on the ability to recognize the end of the repetitions based on the data content.
In its simplest form, you can do this by specifying a regular expression Data Pattern that matches a fixed number of characters that is terminated by reaching the end of the message bit stream.
For example, consider a message with one fixed length field (length 10), followed by another fixed length field (length 20) that repeats indefinitely to the end of the bit stream:
Message Data Element Separation=Use Data Pattern
FieldA Data Pattern=.{10}
FieldB Repeat, Min Occurs=1, no Max Occurs, Data Pattern=.{20}
The following example message contains a fixed length field (length 20) that repeats a variable number of times, and is separated from a second field by the string ";". The pattern specifies a string of 20 characters starting with anything except a semicolon:
Message Data Element Separation=All Elements Delimited, Delimiter=;
SubType1 Data Element Separation=Use Data Pattern
FieldA Repeat, Min Occurs=1, no Max Occurs, Data Pattern=[^;].{19}
FieldB