https://github.com/EOSIO/eos/blob/686f0deb5dac097cc292f735ccb47c238e763de0/plugins/chain_plugin/chain_plugin.cpp#L1043

uint64_t read_only::get_table_index_name(const read_only::get_table_rows_params& p, bool& primary) {
   using boost::algorithm::starts_with;
   // see multi_index packing of index name
   const uint64_t table = p.table;
   uint64_t index = table & 0xFFFFFFFFFFFFFFF0ULL;
   EOS_ASSERT( index == table, chain::contract_table_query_exception, "Unsupported table name: ${n}", ("n", p.table) );

   primary = false;
   uint64_t pos = 0;
   if (p.index_position.empty() || p.index_position == "first" || p.index_position == "primary" || p.index_position == "one") {
      primary = true;
   } else if (starts_with(p.index_position, "sec") || p.index_position == "two") { // second, secondary
   } else if (starts_with(p.index_position , "ter") || starts_with(p.index_position, "th")) { // tertiary, ternary, third, three
      pos = 1;
   } else if (starts_with(p.index_position, "fou")) { // four, fourth
      pos = 2;
   } else if (starts_with(p.index_position, "fi")) { // five, fifth
      pos = 3;
   } else if (starts_with(p.index_position, "six")) { // six, sixth
      pos = 4;
   } else if (starts_with(p.index_position, "sev")) { // seven, seventh
      pos = 5;
   } else if (starts_with(p.index_position, "eig")) { // eight, eighth
      pos = 6;
   } else if (starts_with(p.index_position, "nin")) { // nine, ninth
      pos = 7;
   } else if (starts_with(p.index_position, "ten")) { // ten, tenth
      pos = 8;
   } else {
      try {
         pos = fc::to_uint64( p.index_position );
      } catch(...) {
         EOS_ASSERT( false, chain::contract_table_query_exception, "Invalid index_position: ${p}", ("p", p.index_position));
      }
      if (pos < 2) {
         primary = true;
         pos = 0;
      } else {
         pos -= 2;
      }
   }
   index |= (pos & 0x000000000000000FULL);
   return index;
}

Position of the index used, accepted parameters primary, secondary, tertiary, fourth, fifth, sixth, seventh, eighth, ninth , tenth

index_position 可以以某前缀的名字指定对应多级索引，或者数字（从2开始）