What  means
It marks an embedded object that plain text cannot carry. A 1997 Unicode mailing-list discussion described several vendors independently using a single character as an object anchor, so this was not a one-off pictograph or a single inventor's sign. The purpose was practical: layout, editing, and search could keep one stable position while image or attachment data stayed in a separate structure.
Unicode's 1999 XML guidance draws the boundary: the marker is useful inside an application that still has the object data, but it cannot identify or restore that object in plain text. Inside rich text it is an anchor; after paste or export it is a leftover placeholder.
Why does OBJ show up in someone else's post?
A text handoff exposed an object marker your app did not resolve. A 2019 report on Facebook dictation described the box appearing for browser and Android readers while the iPhone author did not see it. That is a platform-specific visibility split, not evidence the sender typed OBJ on purpose.
Read it as missing content. If the sentence still works, remove the box; if it changes the message, ask what was attached or resend from the source app. Do not read it as sarcasm, approval, or a coded label.
Technical artifact; in a message it reads as broken transfer, not a social signal.
Treat OBJ as missing content, not as a reaction from the sender.
What it means when someone sends it to you
3 readingsDon't confuse it with
1Questions people ask
2Can I tell what was hidden behind OBJ?
No. The character preserves a position, not the object's payload; inspect the original rich-text source or ask the sender.
Why can I see OBJ when the sender can't?
It depends on the client. A rich editor may resolve the anchor, while a plain-text or cross-platform path exposes the marker.
How to type 
\FFFCCodes and encoding1 code points
A visible symbol can contain several code points. JS .length counts UTF-16 units; UTF-8 counts bytes.