The index settings are divided into per-index level and global level. Basically, the purpose of global-level settings is to manage the settings for all indices rather than configuring them one by one. Global-level index settings include circuit breakers, field data caches, node query caches, indexing buffers, shard request caches, and recovery. They are controlled by the so-called indices module. Go to the reference (https://www.elastic.co/guide/en/elasticsearch/reference/master/modules-indices.html) for more details.
On the other hand, per-index level settings are controlled by index modules. Per-index level settings are also distinguished as static and dynamic. Static settings can only be set when the index is created or on a closed index, while dynamic settings can be changed on the live index. The following is a brief description of the per-index level settings...
