An abortable SYNTAX error is a fundamental error in the way that the script is written, such that it could not possibly be run. Examples of abortable SYNTAX errors are forgetting to end a comment or quoted string, using the same label name more than once in a script, forgetting important keywords (such as forgetting to put the type of condition -- HALT, SYNTAX, etc, after SIGNAL ON), etc. The SYNTAX check does not detect all possible SYNTAX errors. Rather, it checks for only those SYNTAX errors which a script cannot handle for itself. So, it is possible for a "syntactically correct" script to encounter a SYNTAX condition (such as memory running out while the script is running). For those kinds of SYNTAX errors, the script will be able to trap and handle them via a SIGNAL ON SYNTAX instruction.
If the script has no abortable SYNTAX errors, then the message "Successful syntax check." will appear in the Status bar.