The raised centre section is more than just cosmetic. It adds some rididity and makes mounting tyres easier.
w/o stands for "wired-on", the correct terminology for what are erroneously called "clinchers". Wired-on tyres have rigid wire or flexible cord hoops encased in the bead. True clinchers have enlarged rubber beads without the wire or cord hoop. In some markets true cllnchers are called beaded edge tyres, which is abbreviated b/e.
HP stands for "high pressure". The rims should have a bead or hook on the inside of the outer edge of the the sidewall. This facilitates retention of high pressure tyres, particularly those with flexible beads.