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It contains information carried in the IP header that may be present but that isn't required to be present, i.e. it's "optional".

For IPv4, see RFC 791, section 3.1 "Internet Header Format", which says:

Options:  variable

The options may appear or not in datagrams.  They must be
implemented by all IP modules (host and gateways).  What is optional
is their transmission in any particular datagram, not their
implementation.


It describes the first few options supported by IPv4; the list of "registered" IPv4 options is on Internet Protocol Version 4 (IPv4) Parameters, under IP Option Numbers.

For IPv6, see RFC 8200, section 4 "IPv6 Extension Headers", which says:

In IPv6, optional internet-layer information is encoded in separate
headers that may be placed between the IPv6 header and the upper-
layer header in a packet.  There is a small number of such extension

Two of the currently defined extension headers specified in this