The indices module allow to control settings that are globally managed for all indices.
The indexing buffer setting allows to control how much memory will be allocated for the indexing process. It is a global setting that bubbles down to all the different shards allocated on a specific node.
The indices.memory.index_buffer_size accepts either a percentage or a byte size value. It defaults to 10%, meaning that 10% of the total memory allocated to a node will be used as the indexing buffer size. This amount is then divided between all the different shards. Also, if percentage is used, allow to set min_index_buffer_size (defaults to 48mb) and max_index_buffer_size which by default is unbounded.
The indices.memory.min_shard_index_buffer_size allows to set a hard lower limit for the memory allocated per shard for its own indexing buffer. It defaults to 4mb.