I am studying the different parts of Instrument Flight Procedures, as they are described by FAA's manuals, such as the Aeronautical Information Manual, TERPS and Performance Based Navigation. For example, during the transition from the feeder phase of an Instrument Approach Procedure to the initial phase, I noticed that under certain conditions the procedure's navigation specification can be changed. For example, in RNAV GPS IAPs, feeder routes can be RNAV 1, while the subsequent initial and intermediate phases switch to an RNP APPROACH specification. Another example is during a SID or STAR procedure, where the navigation specification changes when the route crosses the circle defined by the associated airport and a 30 nautical miles radius.
TERPS 1-8 2.b STAR/feeder and approach legs designed to cross within 30 NM of the ARP more than once do not change to RNAV 1/RNP 1/A-RNP (terminal mode) until the 30 NM limit is crossed for the last time. A departure or missed approach segment designed to cross a point 30 NM from the ARP more than once changes to RNAV 2/RNP 2/A-RNP (en route mode) and remains expanded when it crosses the boundary the first time.
Having said all that, could someone help me to understand under which circumstances it is possible for the navigation specification of a procedure to change and, particularly, if it is more accurate to associate a navigation specification with each leg of a procedure, rather than having one navigation specification that characterizes the whole route.