I have a question about the difference between these two icons. The gray icon appears most frequently throughout my libraries, but I've noticed a blue icon has appeared with an article I entered manually (I'm not sure if the manual entry is related to this difference).

Could someone explain what each icon represents?

Thank you in advance for your help.
https://s3.amazonaws.com/zotero.org/images/forums/u17511932/gld4e2dhv9nlrumdkmeh.png
More details:
https://s3.amazonaws.com/zotero.org/images/forums/u17511932/t5c9bvv11glkbomxywcm.png