UICC stands for Universal Integrated Circuit Card, which is part of the functionality within SIM cards that help identify a device on a particular mobile network. Historically, this has been a static element of SIM cards, meaning that for every SIM card, there is one UICC associated with it.
It is possible that the UICC can migrate to another physical SIM, but if it does, then it is no longer associated with the original SIM, and no other UICCs can be associated with the new SIM. But with a few exceptions (like multi-IMSI SIMs), SIM cards have worked in this way for the last couple of decades.
This is not the case with eUICCs (Embedded Universal Integrated Circuit Card), an 'embedded' UICC, as dubbed by the industry, which has nothing to do with physical embedding.
eUICCs is the software that eliminates the profile lock on SIMs and enables SKUs (single SIM) to hold more than a single mobile network operator profile. Services associated with eUICC are often dubbed eSIM, embedded SIM, digital SIM or iSIM based on their remote provisioning capabilities, form factor or simply the provider's naming choice.
Like any technology, using an eUICC or eSIM comes with a set of challenges, like the extensive testing an OTA network profile requires during implementation. To learn more about how eSIM works and the implications and advantages, look at our selection of blogs on the topic.