Does O, P, or Q stands for different things ?

I have looked at this webpage, but it didn't answer the quesion 
https://www.uniprot.org/help/accession_numbers


Confirmation from UniProt: Indeed accession numbers have no meaning and just serve as unique identifiers.

This is the short answer, and below is some background:


Originally, accession numbers started with P, later with O, P and Q,
but due to the large increase in the number of protein sequences in
UniProtKB, we had to extend the existing accession number format by
allowing the first character to be any of the 26 letters (instead of
only O, P and Q). To avoid assigning accession numbers identical to
those which have been used by the International Nucleotide Sequence
Database, the extension in the first position went along with a
restriction in the third position which can only be a letter. See
https://www.uniprot.org/news/2006/11/14/release


The most recent extension of the UniProtKB accession number format
took place in 2014 to
allow for 10-character accession numbers:
http://www.uniprot.org/help/2014/06/11/release

The "new-style" accession numbers do not have any different meaning at
all, except that they implicitly convey the information that the entry
they identify has been created since June 2014 - however the reverse
is not necessarily true, i.e. not all new entries have 10-character
accessions. In any case, it is best not to interpret ACs in any way,
but just consider them as unique identifiers for a UniProtKB record.
