BREAKUNBOUND causes the current trigger to end. If you are looping over columns in an export where the Unbound property is set to Yes, use BREAKUNBOUND to indicate when the record is fully processed.

For example, your XMLport supports two different file formats. One format contains 5 columns, and the other contains 10 columns. Instead of duplicating the maximum number of columns and writing code that skips any redundant columns, you can set up a single column, make it unbound, and then specify in code when to break to the next element.

Reference