If you double-click a cell and select part of its content, then
Format|Cells... only displays the Font tab. You can then apply bold to
only a part of the cell.

If you seek some automation that will make parts of the cell string
bold, this will require VBA. Something like:

Range("A1").Characters(Start:=1, Length:=3).Font.FontStyle = "Bold"
or

Cells(i, j).Characters(Start:=k, Length:=m).Font.FontStyle = "Bold"

if you are playing with variables.

HTH
Kostis Vezerides