TEXT_BLOCK - Flags all text blocks produced by the SimpleReader.
Vertex
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Flags:
TEXT_BLOCK_SPLIT - Identifies the vertex as a split between text blocks.
OVERFLOW_SPLIT - Identifies that an entire buffer was read without finding a split between text blocks.
The current maximum size of a text block is 64K characters. Text blocks larger than this will be split arbitrarily, and the vertex will be marked with "OVERFLOW_SPLIT".