This is the standard office paper for the more important jobs such as letterheads and reports that need to be well presented. Many firms use 120gsm for a really prestigious impact.
A good 100gsm (and most are - there is not a lot of point in manufacturing a posh paper in an economy version) will take all media, usually with no show-through. Grain is invariably parallel to the long side in the A4 sheets.
Manufacturers are not necessarily specified. There is little to choose, and we stock any one of the more reliable brands, depending on the pricing on offer at the time.