There are various forms of dispatching an a/c but the planned fuel load on the CFP (Computerised Flight Plan) must comply with the Operators Fuel policy as approved by the Operators CAA. Typically this is trip fuel, contingency, alternate and final reserve (30 mins holding at 1500ft overhead altn).
Historically, Contingency Fuel was based on 5% of trip fuel ( climb out of A, cruise, descend into and land at B) and was there to take account of less favourable wind conditions, en-route use of anticing and ATC considerations (lower level than planned). That was when flights were of 4-6 hour duration. With the introduction of long-haul a/c and flight times of 10-13hrs this 5% was one heck of a lotta fuel so authorities agreed to a reduction in the amount of contingency fuel that needed to be carried for dispatch purposes. Hence different methods were adopted and two of which are alluded to in original question.
a) Reclear Planning
b) En-Route Alternate procedure
In Reclear planning there is a specific geographical point defined by Lat/long on the CFP.
With the ERA procedure the "Decision Point" is not defined on the CFP.
The two methods are quite different so let me give an example of "Reclear"
Flight is Singapore to Heathrow. I nominate Frankfurt as my "Reclear" or "Intial Destination" although I have every intention of flying to LHR. I must specify an Alternate airport for FRA and I must also have an Alternate for LHR.
The computer effectively calculates 2 flight plans. The first one is SIN-FRA with normal fuel policy i.e. trip fuel SIN-FRA plus 5% contingency, plus Alternate fuel (say Brussels) plus 30 min holding @ 1500ft o/h BRU. Somewhere before FRA there will be a pre-planned "Reclear Point" Ideally this would be just before Top of Descent into FRA.
Now the computer calculates the second flight plan from SIN to LHR. All CFP's start calculation on ground at Alternate with empty tanks (I'm ignoring exceptions where residual or "unuseable" fuel is taken into consideration) and work backwards towards the origin. From the first flight plan the program will know what is the fuel on board at the Reclear point and assumes that none of the 5 % contingency (SIN-FRA) has been burned off, so it is this contingency that alllows you to "go the extra distance". However for the 2nd Flt Pln the program will calculate 5% of the fuel from the reclear point to the Destination of LHR plus the Alternate fuel (say Gatwick) plus 30 mins holding o/h LGW.
So at the Dispatch stage the a/c has sufficient fuel to fly to FRA with full reserves plus an alternate and holding. It also has fuel to fly to LHR plus altn plus hold but only 5 % contingency from Reclear Point to LHR. When the a/c reaches the Reclear point, if there is sufficient fuel to proceeed to LHR plus the Altn fuel for LGW plus the 30 mins hold then the crew will continue to final destination of LHR. It is desirable (but not mandatory) that at the Reclear point the 5% contingency (rclr point to LHR) is still on board.
So the Reclear Point is a specified, pre-determined position. If memory serves the optimum point for this position is 8/9ths of the distance origin to final destination. This procedure only works with sensible selection of Initial Destn and its associated Altn. The ATC plan is filed to the final intended Destn, in this case LHR.
The other form of Dispatch (En-Rte Altn Procedure) is somewhat different. Again I will use SIN-LHR as an example.
It is not necessary to carry the full 5% contingency LHR-SIN but I do need to have something up my sleeve to guard against things going pear-shaped so I nominate an En-Route Alternate (in this case I will again use FRA) but my contingency fuel will be the greater of
a) 5% of the fuel from the En-Rte Altn to the Destn. or
b) The fuel for 15Mins holding o/h Destn (LHR) in ISA conditions taking into account any a/c performance degradation.
Item b) is a means of carrying reduced reserves but having a "quantifiable" amount. Almost invariably item b) will be the specified "Contingency). There are various caveats with regard to determination of the En-Rte Altn such as the Wx must permit a landing if required and the Wx criteria are those for an Altn rather than a destn and must be sustainable for 1 hour prior to and after likely arrival time. Rules govern where it must lie in relation to track (one being a min of 150nms from Destn.)
Now for CRUNCH Time!!
In flight a check must be made at the DECISION POINT to establish that sufficient fuel remains to continue to the Destination plus Altn Fuel Plus 30 mins Holding. If insufficient then a landing should be made at the ERA ..... OR A CLOSER DESTINATION planned. (What a lovely get-out!)
The DECISION POINT is not a pre-determined Lat/Long. It is the point at which the a/c would deviate from planned track or level in order to descend into the ERA.
Consider the case where the ERA is "on-Track" Origin-Destn and consider as typical descent distance for jet of 120 nms. Some think that the Decision Point would be 120nms before the ERA, some think the DP is Abeam the ERA but in fact the captain could elect to make his Decision 120nms past the ERA.
The above are "Dispatch Issues" and not to be confused with PNR's Crit Points or the like mentioned in other responses. Obviously Depressurisation is a consideration in Flight Planning be it for ETOPS or 3/4 jet over large expanses of water but the terminology used in original query regarding Reclearance and Decision points do not relate to these scenarii.
Hope the above clarifies.
