People are talking about RR as buying units --- but I'm not buying it. The new ship build rates are likely very, very low. Let's take some perspective from the best "balanced" war in the B5 Universe, the Narn War of Retribution (all references sourced from AoG's manual on the subject, which is pseudocanon.)
The whole war lasted, from opening salvoes as Quadrants 1 and 21, 26 weeks (one half a year). During that period, the Centauri reinforcements took the form almost solely of reactivations from mothballed fleet units. I see no references to new construction during the war on most of the ships. Similarly, the text refers to the Narn starting their END (Emergency Naval Development) program almost immediately after winning their liberation in 2209. It is reasonable to assume that the early designs (which we do not see) and early expansion took up much of the Narn efforts likely through 2220 or so.
This leaves approximately 30 years for final END development efforts. The Narn do not seem to have plotted this war in advance; they were not bashful about the possibility, but it was not a war of aggression. From this, I conclude that the END effort likely continued at a relatively constant level of development over the previous 30 years.
Additional wartime mobilizations were likely needed to provide the supply, fuel, resources, and manpower necessary for conquest. As a first order of estimation, it is likely that the Narn Empire was capable of reconstructing perhaps 5-7% of their total naval deployment through END in a given year. Given that the Narn War of Retribution lasted 26 weeks, this means that a total of 3% of all Narn vessels deployed during the War of Retribution were constructed during the war. Short-term acceleration is possible during the campaign, but unlikely. This is a shockingly low level of reconstruction compared to attrition losses.
Note that this speaks to captial ships. Non-jump capable vessels and fighters which can be isolated may have been able to be constructed, but could not be leveraged. We do not, therefore, know the replacement rate for small vessels and surface installations.
So, if you can't expect in a Grand Strategy game to rebuild entire Navies .... what the heck are RR?
I contend that RR are an abstraction of the advantages of supply, mobilization, lines of communication, and fuel efficiency. It also reflects supply and Space Superiority necessary to perform in-system repairs to return vessels to fighting condition. In the case of Auxilliary Craft, it is likely their replacements, as well.
Given that the Carrier is likely the only vessel that can take losses and return to a fight, this means the Fleet Carrier and its mates are major elements of Strategic warfare of Major Powers.
Note this reflects insights to the Narn economy. Centauri have a slightly differenct economic system; the large quantities of mothballed vessels provide a reinforcement pool that few (if any!) other nationalities have. EA seems to rebuild navies at a shocking rate, given the substantial losses during the Dilgar War (2231-2232), total reconstruction of forces following the EA-Minbari War of 2245-2247 (wherein some substantial reinforcements must have been deployed), and then re-engaged in the Clark Civil War of 2262. This suggest the Earth Alliance is able to reconstruct major fleets within a 15-20 year window, for a more substantial base 6% construction rate, similar to the Narn. However, given the enhancement in techonology over the same timespans including the development of the Omega-class, EA construction rates may be even higher. The relatively slow speed of the Minbari advance may be an insight into deficiencies in their logistics capabilities, construction or otherwise. The Minbari construction rate is also likely shockingly low for such an advanced race.
I'll stop rambling. My point is that RR's aren't replacements. We have to show exactly what they are, however.