The "600" is 600 mAh (milliAmp Hours) or 0.6 Amp Hours. It is referring to the total capacity of the battery cell. A 600 mAh battery (assuming it is an honest and accurate description and at optimum functionality) will have 50% more capacity than a similar 400 mAh capacity cell.*
*Generally, I would assume, that more capacity would be better, but there is one reason I imagine that a capacity closest to actual expected use may be preferred. NiCd have a terrible "memory" issue. They tend to lose capacity and need to be "refreshed" with full charge and discharge in order to return to optimal capacity. (Often a few times.) Thus, in these limited situations, it may be potentially preferable to have a battery that has just about the amount of required capacity (e.g., a dusk to dawn light), so that it doesn't suffer as badly from the NiCd memory effect. While NiMH batteries also have a similar memory issue, it is not, I believe, anywhere close to the severity of NiCd. (Perhaps no more than 10% of the severity) So, if it were me, I would probably consider using better, higher capacity NiMH. See below.
For additional reference: I don't know about the application of rechargeable batteries for solar usage, but for my household uses (e.g., TV remote) I prefer the (now) more common NiMH (Nickel metal hydride) batteries to these NiCd (Nickel-Cadmium) batteries for their greater longevity. Typical AA NiMH batteries have 2Ah (2000mAh) capacity and are widely available. I would imagine they are interchangeable, but unsure if there is any preference for NiCd in outdoor situations (e.g., solar, as in here). Additional info: Alkaline AA have an average, minimal working voltage of 1.5v, NiMH have 1.2, I believe NiCd is also 1.2 (at least in these AA, AAA type form factors). Lithium variations are of an entirely different sort, and while they can range greatly, are typically around 3.7v, with lower or greater capacity and generally require more elaborate charging equipment due to their more volatile chemistry.
Date published: 2017-04-17