This is an ENC meta object called Quality of data (M_QUAL) that has attribute Category of zone of confidence in data (CATZOC). The categories are A1, A2, B, C, D, and U. They are defined in Introduction to Electronic Chart Navigation, Figure 2.4-7 and related discussion.
It is not often the data are less than A1, but we learn this in the ENC as each category has a unique symbol, and indeed a common issue is these symbols, shown below, clutter the chart, so we usually want to shut them off. They do not show in a Standard display, so when they show it means we are in a Custom, All, or Detailed display mode with Quality of Data turned on. To clear up the chart, turn that off.
The 6 star symbol is CATZOC = A1; 4 stars means CATZOC = B.
To see details of all zones, see attribute CATZOC at www.s-57.com