If you use the full name when using a trade name, such as a food business department, although "a" is already a registered trademark of others, it does not constitute infringement;
If the trade name used is not full name or prominent, it may involve infringement. For example, the food division is abbreviated as "XXX", or "XXX" is particularly prominent in use.
Of course, if someone else's registered trademark already has a certain popularity and reputation, you deliberately use it as your own trade name for the sake of famous brand and hitchhiking, which will inevitably involve infringement.
According to the provisions of Article 57 of the Trademark Law of People's Republic of China (PRC), the following acts are infringement of the exclusive right to use a trademark:
Article 57 Any of the following acts is an infringement of the exclusive right to use a registered trademark:
(1) Using the same trademark as its registered trademark on the same commodity without the permission of the trademark registrant;
(2) Without the permission of the trademark registrant, using a trademark similar to its registered trademark on the same kind of goods, or using a trademark identical with or similar to its registered trademark on similar goods is likely to cause confusion;
(3) selling goods that infringe upon the exclusive right to use a registered trademark;
(4) Forging or unauthorized manufacturing of registered trademark marks of others or selling forged or unauthorized registered trademark marks;
(five) without the consent of the trademark registrant, the registered trademark is changed and the goods with the changed trademark are put on the market again;
(6) Deliberately facilitating the infringement of the exclusive right to use a trademark of others and helping others to commit the infringement of the exclusive right to use a trademark;
(seven) causing other damage to the exclusive right to use a registered trademark of others.