Each BAND is separated into B - W - S grades (beginning, working within and secure). Your child might be well on their way to the next grade (or 'step') and so could be assigned a plus '+' grade.
E.g. Year 3 - A child could be a 3B, 3B+, 3W, 3W+, 3S or 3S+
Typically, a child would be a 'B' at the beginning of a year, 'W' within the year and possibly an 'S' at the end of a year. It's worth remembering though, that all children progress at different rates. Our 'expected' attainment grade at the end of the academic year is typically a W+ or S. If your child is awarded a W+ or S in July, then they are attaining at the expected level. S+ would be higher than expected, whilst a W grade or lower would indicate that your child is achieving at below the expected level.
Take a look at some exciting writing from pupils who are working at the nationally expected levels (or above) in their year. This will give you an idea of where a child should be aiming at, for their age.