The disk icon will “fill up” as the total percentage of all log files in all databases increases.
This is the current Log Used space in all log files as a percentage of the current size of all log files.
Note that the all figures in the disk icons are accumulated over all databases so that a Percentage Used figure of around 0% does not necessarily mean that all logs have plenty of space left. Also, some files may be configured to AutoGrow, so if this figure is approaching 100% it does not necessarily mean you are almost out of space.
The Fixed Log File Used or the Databases - Dynamic Log File Growths Remaining alarm is raised when any database is running out of log space.