MediaWiki:Apihelp-wbsetclaim-param-index/en

The index within the entity's list of statements to move the statement to. Optional. Be aware that when setting an index that specifies a position not next to a statement whose main snak does not feature the same property, the whole group of statements whose main snaks feature the same property is moved. When not provided, an existing statement will stay in place while a new statement will be appended to the last one whose main snak features the same property.