Which type of signal is described as a "black X" and is an absolute stop indication?

The signal described as a "black X" and functioning as an absolute stop indication is a dwarf signal. Dwarf signals are typically low-mounted signals located near the tracks, designed for specific instructions or indications to train operators, often at points like terminals or in situations involving intricate switching maneuvers. The specific "black X" representation is a clear visual cue indicating that the train must come to a complete stop; it signifies that there are no further actions permissible until the signal is cleared or additional instructions are provided.

Dwarf signals are particularly useful in locations where space might be limited or where a signal needs to be placed in closer proximity to the ground for better visibility for the crew in a stopped train. Other signals, such as home signals, approach signals, and marker signals, serve different functions. Home signals typically indicate where the train can enter a block, approach signals advise trains on conditions ahead, and marker signals might indicate specific railway landmarks rather than providing stop instructions. Thus, the black X is specifically recognized as an absolute stop indication through dwarf signals.