The most common (and preferred) type of displayed equation in Physical Review is a narrow, indented single-line equation, with an equation number on the same line. Try to set as many equations as you can in this way. Because some equations are longer than this type of styling will allow, there are other types of displays to accommodate longer equations.
Equations can be set flush left with the margin if they do not fit in the column; dropping the equation numbers to the line below the equation will also help and is handled automatically by REVTEX for single-line equations. Breaking the equation into multiline format may be necessary for very long equations.
If an equation needs to be broken into more than four lines, it should be set in a wide column for ease of reading, using the \widetext command. The author should return to \narrowtext as soon as possible after one or more very long equations.
In Sample Compuscript A, we have illustrated how to obtain each of the above.