China did not participate in the voting as it has a long-standing policy of not participating in votes on resolutions of which it does not approve. The reasons for not participating or abstaining:
the Chinese have been reluctant to use their veto power, opting
instead for abstention and non-participation in votes. In this
way — typical of Beijing’s behavior in the UN as well as reflecting its
cultural legacies — the Chinese are able to send a message and yet avoid
the necessity of taking sides and alienating allies.
Further,
As a rule, the Chinese prefer that conflicts be settled by the parties
concerned or, as a last option, by local and regional organizations,
without external intervention, including that of the UN or the
International Criminal Court.
China did also not participate in voting on Resolutions 338, 339, 340, 341, 346 and 347, all of which dealt with the Arab - Isreali conflict around the time of the Yom Kippur War.
The UNSCR article link provided by Pete Leaman in his comment makes clear that China disapproved of Resolution 344, mentioning that China 'dissociates itself". This is also mentioned in Resolutions and Statements of the United Nations Security Council (1946 - 1989). This same source also mentions that France abstained as it had 'reservations' about Resolution 344. There is little else on the reasons why the US, the UK, the USSR and France abstained. This Security Council report only says:
...clearly all of the permanent members of the Council had problems with
this resolution. Perhaps they saw it as a challenge to their
leadership on the Middle East issue.
None of the (now many) sources I've consulted explain the difference between abstaining and non-participation in voting, but we can perhaps surmise that the former indicates a lack of full agreement (without being completely opposed) whilst the latter is a stronger message indicating that China does not think that an issue should even be voted on. China is particularly sensitive about what it perceives as interference in the affairs of other states. Christopher Holland, in Chinese Attitudes to International Law, states:
In 1953 and 1954, Chinese Premier and Foreign Minister Zhou Enlai set
out the Five Principles of Peaceful Coexistence, which now form part
of the Preamble to China’s Constitution. These principles include
mutual respect for sovereignty and territorial integrity, mutual
non-aggression, and non-interference in other state’s internal
affairs. Throughout the Cold War, China practiced strict adherence to
these principles, for example, by refusing to participate in Security
Council votes on peacekeeping, or to contribute to UN missions.
Holland also notes that:
Since the end of the Cold War, Chinese foreign policy has shown
“greater moderation, engagement, and integration”.
Perhaps related to at least some of the above is China's belief that the General Assembly should have greater power compared to the Security Council. Historically, China has been reluctant to use its veto on the Security Council: only 3 times by 1989, compared to 114 vetoes from the Soviet Union, 67 by the US, 30 by the UK and 18 by France. Since then, China has vetoed more frequently but not usually on issues which do not directly related to Chinese interests. Nonetheless, China strongly believes in having the power to veto.
