I'm not sure we have enough information, in the screenshot, to fully understand your configuration. However, what I can see there is that you appear to have one "pool" which is "A". That pool appears to have only one disk group "dgA01" assigned to it. The disk group is just shy of 5.4TB and has 635GB free. This probably means your "volume" (which we cannot see) is only currently using about 4.8TB. Whether it is a 4.8TB "volume" or a larger "volume", that just hasn't filled up yet, I cannot tell. By default, you can overcommit storage so there is nothing stopping you from increasing the size of the "volume" (through CLI or GUI) to whatever you want. You could make it 32TB right now if you wanted and then you just need to follow the operating system instructions for recogning the additional space (usually a "diskpart extend" for windows). However, you'll only ever be able to write as much space as the volume has available in the pool. So, in this case, the volume will stop accepting writes in about 600GB.
So, what you probably want to know is "how do I add more space to the pool?" In this case, you purchase more disks and add them as an additional disk group to Pool A. The system will then level out the data across those two groups so that performance increases with the additional spindles. The recommnedation is that all of your disk groups be configured the same within their disk type. So, in this case, you would want to purchase another 10 disks and add them to the pool as a RAID 5 disk group (to match what you have). In actuality, you'd want to order 11 disks and mark one as a gloabl spare.
Finally, you could purchase a tier of (larger/slower) nearline disks and configure them as an "archive" tier within the pool. This means that your less frequently used data would be moved down to the NL disks and the hotter data would stay on the 600GB "standard tier." This decision shouldn't be taken lightly; you need to understand your workload first as you could potentially introduce a performance bottleneck depending on your use case. Finally, you can add SSD disks as a "performance tier" to move your hot data up (rather than cold data down). Adding SSD to the pool will require a "performance tiering license" if you don't already have it. Or add both SSD and NL, the sky is the limit.
In short, add some more disks to the pool, expand the volume from the MSA side, and then expand it from the host side.