PlayerItem is an EQ_PlayerItem struct contained within the main struct. It's defined at the top of the file as a 10 byte struct (It has offsets from 0x0-0xA). You can get the real offsets inside the main struct by adding 0x114 (it's position in the main struct) to the EQ_PlayerItem offsets.
For example, the offset of PI.Countflag = 0x114 + 0x0
PI.Level = 0x114 + 0x1
PI.Count = 0x114 + 0x2
..
PI.MagicMask = 0x114 + 0x6
What they mean though, I'm not entirely sure. Most of the values in the PlayerItem struct are duplicated, PI.Spell, PI.Level and PI.Effect are the same as Common.SpellId, Common.Level and Common.Effect. I just grabbed the file from Hackerquest, I don't pretend to know what they all mean
K.