You can insert a construct around a highlighted region. The highlighted region becomes one of the components of a construct. For example, if you have a region containing text that you would like to set in Roman, first highlight it, then choose Format > Font > Roman. If you want to turn that region into the numerator of a fraction, highlight it and choose Constructs > fraction. The Equation Editor automatically chooses the component that the region becomes, so it is not possible to highlight a region and make it into the denominator, for example. To do this, you would have to use Cut and Paste.