It is the code for late, these are the SIMS codes which shows a child is in for the register or why they are not.
L - late before registers close - meaning marked as "present"
O - Unauthorised absence - meaning unauthorised absence
U - Late after registers close - meaning unauthorised absence.
It has been a while so I can't remember when registers close, it is decided by the school. Here is the full list of SIMS codes. Random school chosen
https://millgreen.org.uk/attendance-codes-descriptions-and-meanings/
Look stop with the whole 100% attendance shit. We had 90% of a year group wiped out with norovirus, children were unwell, parents sent them in and then a child threw up on my child's shoes. So either a child was actually ill, like my own after this or some parents kept their children off school as it happened the last week of term in December and people didn't want ill children near Christmas.
I would suggest it would be better to have a 98% and above award system and I would contact school for that, far more positive. A child cannot help being ill. At secondary, ours had a 100% attendance and then a 98% attendance plus a special we know you have a chronic health condition one too. All of them came with rewards.
The fact is, your child was late. It happened to mine, year 6 never been late in his life, traffic lights were out at a major junction which meant a lot of children came in late. Ds was mortified because he also missed his in school duties.