This is the battery of choice for HO-scale narrow-hooded diesels (GP series, for example). It replaces BPS-300 and BPS-500 batteries which will be discontinued.
This adds a larger cell to the BPS battery range. It is intended for use in steam loco tenders and in S-scale diesel locos.
There is no change here. This is the most popular BPS battery used for HO-scale full-width diesels (E and F series, for example) most steam loco tenders and all larger scales (On30, On3, S). The new BPS-1000 is expected to replace it in some of these applications.
"P" indicates cells are connected in parallel. Voltage is same as single cell (nominally 3.7 volts) and capacity adds so a 2P-420 battery provides 840 mAh.
25C indicates cell is rated for high discharge; specifically, 25 times cell capacity. That is, 25 x 420 mA equals 10.5 amps from one cell and 21 amps from 2 cells in parallel. This should remove any doubts regarding the importance of embedded fuse protection.