The Web versions are designed for embedding on web sites. To keep them as small as possible, they lack most of the symbols found in the Print/PDF version, have no Alternative Fractions, and are unhinted.

They contain some OpenType features, such as Petite Capitals (x-height Small Capitals), Capitals to Petite Capitals, Standard Ligatures, Discretionary Ligatures, Superscripts, Subscripts, Scientific Inferiors, Ordinals, OldStyle Figures,  Fractions, and Kerning.

They have the same fontnames as the Print/PDF versions, it is not possible to install them simultaneously.  So make sure that you have the appropriate version installed for Print or Web publishing. 

The WOFF (Web Open Font Format) versions are the same as the OpenType Web versions, but compressed. The disadvantage is that they cannot be installed in Windows and cannot be embedded automatically using Serif WebPlus X8.  You will need to add suitable @FontFace code manually to make use of them.