What is 'FT' means in Nginx NGX_HTTP_UPSTREAM_FT_* macro/const

There are some macro about nginx upstream and What is 'FT' means in these definitions?

#define NGX_HTTP_UPSTREAM_FT_ERROR           0x00000002
#define NGX_HTTP_UPSTREAM_FT_TIMEOUT         0x00000004
#define NGX_HTTP_UPSTREAM_FT_INVALID_HEADER  0x00000008
#define NGX_HTTP_UPSTREAM_FT_HTTP_500        0x00000010
#define NGX_HTTP_UPSTREAM_FT_HTTP_502        0x00000020
#define NGX_HTTP_UPSTREAM_FT_HTTP_503        0x00000040
#define NGX_HTTP_UPSTREAM_FT_HTTP_504        0x00000080
#define NGX_HTTP_UPSTREAM_FT_HTTP_403        0x00000100
#define NGX_HTTP_UPSTREAM_FT_HTTP_404        0x00000200
#define NGX_HTTP_UPSTREAM_FT_HTTP_429        0x00000400
#define NGX_HTTP_UPSTREAM_FT_UPDATING        0x00000800
#define NGX_HTTP_UPSTREAM_FT_BUSY_LOCK       0x00001000
#define NGX_HTTP_UPSTREAM_FT_MAX_WAITING     0x00002000
#define NGX_HTTP_UPSTREAM_FT_NON_IDEMPOTENT  0x00004000
#define NGX_HTTP_UPSTREAM_FT_NOLIVE          0x40000000
#define NGX_HTTP_UPSTREAM_FT_OFF             0x80000000

FT in nginx vocabulary means File Type. See this thread for more information - https://serverfault.com/questions/390757/how-to-set-vim-filetype-when-editing-nginx-config-files/782503#782503