Available if airMAX is enabled. airMax Capacity (AMC) is based on airtime efficiency. For example, if you have one client with a low data rate or you are using a 1x1 device (such as Bullet or airGrid) alongside other clients that are 2x2, then it will use up more airtime (slots) for the same amount of data, reducing time (or capacity) for other clients. The lower the AMC, the less efficient the AP is. If you only have one client, this may not matter, but when you have many clients (for example, more than 30), then AMC becomes very important, and you want it to be as high as possible.
If you are looking at the client, AMC shows the theoretical capacity of that client, based on current TX/RX rates and quality. AMC is a percentage based on what the maximum performance would be if the link were perfect. Clients with poor airtime efficiency can negatively affect other clients by taking up more airtime while transmitting at lower speeds. For example, client A is at MCS 12 (78 Mbps) because of low signal. The client could theoretically do MCS 15 (130 Mbps), so AMC is based on the ratio of current rate/maximum rate (78 Mbps divided by 130 Mbps), which is 60%. In a similar fashion, a 1x1 device will always have a maximum AMC of 50%, because it provides half the performance of a 2x2 device.
If you are looking at the AP, then AMQ and AMC are averages of all clients’ values. If you want to discover what is lowering your values on heavily populated APs, single out the weak clients. You can either use airControl™ (recommended), or you can go to each client individually. Try to upgrade to a higher-gain antenna (to allow a better data rate).