...
Array index - index of an element in an array, if the attribute is an array. In the Ethernet/IP protocol, an array is indexed from zero, so if the Item count is equal to 3, indices 0, 1, and 2 are allowed.
Note: if Array index + 1> Item count, then Array index + 1 element will be read instead of Item count.
Note: instead of using an index, it is possible to specify the index directly as part of the symbolic name, e.g. "Arr[3]" or "Arr[5]". Then such I/O tags will be read separately. However, if an Array index is configured, the I/O tags will be read together (the Arr object and the number of elements 6 will be read, i.e. indices 0 to 5). Thus, it makes sense to use an Array index if we want to read several elements from the beginning of the array and write the index as part of the symbolic name if we read a single element with a high index (e.g. "Arr[100]").
Bit (1..n) - if an attribute type is an integer (unsigned/signed) number (BOOL, SINT, INT, DINT, LINT, USINT, UINT, UDINT, ULINT, DATE, TIME_OF_DAY, BYTE, WORD, DWORD, LWORD, ENGUNIT, and arrays of these types) it is possible to enter a specific bit (1 to 64, depending on the number of bits for a specific type).
...