In Nier Automata, when equipping plug-in chips, I noticed that some of the plug-in chips have a black diamond after the name, for example "Melee Defense +2 ◆". At first, I thought it meant that the plug-in chip was equipped, but I then noticed it on other chips that I know I didn't have equipped. Is there any significance behind the diamond? If so, what does that black diamond mean?
The diamonds signify that the plug-in chip has a significantly lower memory usage cost to equip compared to abnormal version of the same chip. I've had chips with diamonds cost 10-15 less memory to use before, so they're definitely worth looking out for.
If you combine chips with a diamond next to it, make sure you combine it with another with a diamond, to keep the cost down. If you combine it with a regular version of the chip, it may slightly lower the cost from what it would usually be, but it will lose the diamond status and cost more than it would if it had the diamond, unless the regular chip has the minimum cost without the diamond, as explained here:
To explain further, an initial level diamond ◆ chip, or level 0, will have a cost of 4 and the basic equivalent will have a cost of 5-10, however the 5 cost can be fused with the diamond chip to get a diamond ◆ chip +1. The basic chip to retain diamond ◆ rating will always cost 1 more then the diamond ◆ version.
The same however is not true for even upgrade ranks (Ex. +2, +4, +6,) as the formula for fusing chips will cause an extra increase and instead of gaining a diamond ◆ of the next rank will give a minimum basic. (Possibly useful if you have the chips you'd need for higher ranks but not enough of lower ranks.)
Note that, as mentioned in the article, this only works on odd numbered upgrades.