Question on raw material semi finished and finished goods

Hi,

Will it be always that a raw material will have a purchasing view in Material master data and no sales view?

Likewise for a finshed product we can have only sales view in a material master data and no purchasing view always?

Also I would like to know a difference between a raw material and semi finshed goods with an example.Basically how can we differentiate if a material can be used as a raw material or a semifinsihed material?