While the term Flag is primarily a programming term, it is commonly used term in the Neptunia series and holds a variety of meanings.
Meanings
Programming Term
The term "Flags," from their use as landmarks, is used in programming to mean a requirement to process an action. If the requirement's cleared, one says "the flag is true" or "the flag has been raised." When it's not met, one says "a false flag falls."
Derivative 1
Originally a programming term, it's come to be used to mean "progress in a relationship," "omen of disaster," and other divergent meanings. A famous flag is "speaking optimistically and holding onto hope when things are dire." Since most die after this, it's known as a "death flag."
Derivative 2
When a flag doesn't go as expected, or when a person stop the outcome themselves or ignores it, it's known as "breaking the flag." Those who do this excessively are called "Flag Crushers." The usage has diverged from the meaning, and programmers in nations outside of Japan say using the word breaking isn't right.
Flag Item
In Gamindustri, once in a great while, the usually intangible concept of "flag" materializes into an item. The item is categorized as "Flag Item", and one receives beneficial functions by obtaining one. Previously, adventurers and scouts raised and lowered it within dungeons to their benefit. However, as it is much more convenient as an item, most people pull it out and carry it away now, so you rarely see one in the the wild.