[Vuejs]-Vue JS what does # do inside a <template> does in v-data-table?

According to official docs

v-slot has a dedicated shorthand #, so <template v-slot:header> can be shortened to just <template #header>. Think of it as "render this template fragment in the child component’s ‘header’ slot"

So <template #[`item.amtv_start_sales_date`]="{ item }"> is originally written as <template v-slot[`item.amtv_start_sales_date`]="{ item }">