LINE_BREAKER and BREAK_ONLY_BEFORE are both props.conf settings, and they're used in different parts of the parsing / indexing process.
You can see a detailed chart of this on the Splunk Wiki. But LINE_BREAKER defines what ends a "line" in an input file. By default it's any number of CR and LF characters. (Depending on your format of your input, this could need to be altered for correctness, or if your log format can be separated into events by a simple regex, LINE_BREAKER can be altered to find the event boundary, and SHOULD_LINEMERGE can be set to false to skip the next step of the process).
During the next phase, Splunk takes the individual lines and combines them back together to form events. (Certain log formats may have multi-line events, especially stacktraces). BREAK_ONLY_BEFORE is one of many attributes used to determine where the event boundaries are.
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