Complex sentence: Cardinal Bishops of suburbicarian dioceses are also titular bishops, but they hold titles to existing dioceses, namely one or more of the suburbicarian sees, but while they do not govern their see they are obliged to give it their patronage.
Split into simpler sentences: Cardinal Bishops of suburbicarian dioceses are also titular bishops, but they hold titles to existing dioceses. Suburbicarian cardinals hold the title to their suburbicarian see, but while they do not govern their see they are obliged to give it their patronage.