256w vs. 256v encoder/decoder

[mayeranalytics]

What is the difference between p4nenc256w32 and p4nenc256v32? The v stands for vertical, I believe. I cannot find any information about what 'vertical' means, nor what the 'w' variant might be. Looking at the decoder, instead of a corresponding decoder p4ndec256w32 there is p4nddec256w32 ('n' vs. 'nd'). Could you please document what these function are and how it makes sense that the 'w' encoder and 'w' decoder do not quite mirror each other? Thanks.