R28 : Rebalancing example, required, existing, security

LBriscoe

V4, P30~31

On the top of P30, it is said : portfolio rebalancing is required to keep the portfolio DURATION syncronized with the horizon date. Then in 4.1.1.5 and Example 6 & 7, the rebalancing of the "DOLLAR DURATION" is introduced.

My questions :
Which shall be rebalance ? DURATION or DOLLAR DURATION ? How to rebalance the DURATION (not introduced in the text) ? In example 7, what does it mean by : We choose to rebalace using the existing security proportions of "one-third each" ?