Read-only, though arguably it should be correcting by default.
The latter, block group usage is the value used in -dusage, it means that it will only balance data blocks groups with usage under the set percentage, metadata is not balanced as it's usually not needed, or even recommended unless there's a specific reason to do it.
A few more observations:
50% should be a good default for most use cases, pools that have many extents per block group with frequent changes/deletes to some might need a higher value to keep a good data usage ratio, for most cases anything above 75% should be fine, it's this value:
So check that value after the scheduled balance runs, once at month should be enough for most cases, and if the usage ratio drops below 75% you can increase the "block group usage" value a little, usually no need to go very high, the higher it's set the more data will be balance resulting in unnecessary wear for flash devices, with a value of 100 all data will be balanced, i.e. re-written, same as a full balance, except full balance also balances the metadata.
Also note that the usage ratio by itself is not critical, it should be viewed in conjunction with the available unallocated space, as long and there's some it's not a problem, but it's good practice to keep it under control, if the pool is run close to full, or frequently filled up it might be good to aim for a higher data usage ratio.