Number combination: A number combination is a specific sequence consisting of a series of numbers. Numbers can represent a certain meaning, such as date, phone number, password, etc. Written in 539 at the door, 539 represents some specific information or code, and the specific meaning needs to be explained according to the context.

2. Symbols or marks: Symbols or marks are patterns, symbols or marks used to express certain information, identity, ownership or recognition. Numbers, letters, patterns or other specific symbols will be used to mark houses, buildings or personal property. Written in 539 at the door, 539 is some kind of sign or logo, and its specific meaning needs further understanding.

3. Pranks: Pranks refer to intentional, humorous and malicious behaviors aimed at teasing, fooling or hurting others. In some cases, digital signs are written on doorways as part of practical jokes.