The concat action allows you to combine the values of multiple fields into a single field using a specified format string. It takes two arguments: key, which specifies the target field to which the concatenated value will be written, and format, which specifies the format string to use.

The format string can contain placeholders in the form %FIELD_NAME%, which will be replaced with the corresponding field values when the action is executed. For example, in the YAML file you provided:

actions:
  label_formatting_nl_be:
    action: concat
    key: label-nl_BE
    format: '%label-nl_BE% (%code%)'

The concat action will combine the values of the label-nl_BE and code fields using the specified format string, and write the result to the label-nl_BE field.

Input:

item:
  - label-nl_BE: "Product A"
  - code: "123456"

YAML file:

actions:
  label_formatting_nl_be:
    action: concat
    key: label-nl_BE
    format: '%label-nl_BE% (%code%)'

Output:

item:
  - label-nl_BE: "Product A (123456)"
  - code: "123456"

In this example, the concat action combines the values of the label-nl_BE and code fields using the specified format string, and writes the result to the label-nl_BE field. The original value of the code field is preserved.