Thanks.
SD cards can be used in two ways: either to extend the volume of the phone’s internal memory, or as a completely separate storage area, just like a USB stick or PC disc drive. This second option is often referred to as “Portable” or “Removable” because you can remove the card from the phone, for example to transfer the files it contains to some other device.
See the Forum SD card Guide for details, and also here.
If you’re not sure of the way your card is being used, you can connect the phone over USB to a PC or Mac. On the computer’s file explorer, you will see the SD card as a storage area separate from the phone’s internal memory. You can also check using apps and settings on the phone, but connecting to a computer is probably the clearest and simplest way.