GS1-128, formerly known as UCC/EAN-128 Symbology, is a variant of the Code 128 Symbology.
The UCC EAN System was established by Uniform Code Council (UCC) in 1973.
In 1977, the European Article Numbering Association (EAN International) designed a compatible system for use in Europe.

The pattern of using FNC1 right after start character is reserved for GS1-128 (UCC/EAN-128).
According to the specification, a GS1-128 (UCC/EAN-128) symbol has the following nominal dimensions:

Space is excluded from encodable charset.
The maximum data characters can be encoded is 48.
Including quiet zones, the maximum physical length is 165 mm (6.5 in.).
The minimum height of the symbol is 32 mm (1.25 in.).
The Human Readable Text of the data must be shown below the symbol. Start, stop, function character and check digit are not shown.
Application Identifiers (AIs) define the interpretation of data.
GS1-128 (UCC/EAN-128) Symbology Structure

A Code-128 Start character (A, B, or C).
FNC1
Application Identifier (AI)
Data
Check Digit
Stop Character

It's a smart and simple solution to use Barcodesoft Encoder to generate GS1128 barcode.
Just put parenthesis before and after each Application Identifier(AI) as shown below, Encoder will add FNC1 automatically when it is needed.
(02)12345678901234(10)12345678901234567890(15)090422
A FNC1 will be added at the beginning of the string. Another FNC1 will be added before the third AI (15) because the previous AI (10) is a varied-lengthed AI. Scanner will need this FNC1 to decide the end of previous Application Identifer's data.
AI (10) could have 1 to 20 alphanumeric characters as data.

When this barcode is scanned, user will get the following string
[C102123456789012341012345678901234567890#15090422
The first FNC1 is interpreted as [C.
The second FNC1 is interpreted as ASCII 29 GS.