slog, alog and ulog files are one the important log files, to check the heart beat of the SAP server. We can understand by reading this logs files, Why the error has come and we can get an idea to overcome with the error. While doing the version upgrade, we have to check this log files. This will help you to understand the background processing.
SLOG: This log file provides the information on steps started by tp which have already run or in the process of running. SLOG is updated by tp.
ALOG: This log file provides the information what actions have performed when tp has started. ALOG also updated by tp. You can find out the tp action in Transport phase log (tp log).
ULOG: All tp calls executed by the system while doing the transports or patching are contained in ULOG with the relevant call parameters.