I am not sure if it is mentioned in documentation but when you download bowtie2, in source code (file - sam.h) all fields are mentioned-
// Following are Bowtie2-specific
bool print_yf_; // YF:i: Read was filtered out?
bool print_yi_; // YI:Z: Summary of inputs to MAPQ calculation
bool print_ym_; // YM:i: Read was repetitive when aligned unpaired?
bool print_yp_; // YP:i: Read was repetitive when aligned paired?
bool print_yt_; // YT:Z: String representing alignment type
bool print_ys_; // YS:i: Score of other mate
bool print_zs_; // ZS:i: Pseudo-random seed
bool print_xr_; // XR:Z: Original read string
bool print_xt_; // XT:i: Time taken to align
bool print_xd_; // XD:i: DP problems
bool print_xu_; // XU:i: ungapped alignment
bool print_ye_; // YE:i: streak of failed DPs at end
bool print_yl_; // YL:i: longest streak of failed DPs
bool print_yu_; // YU:i: index of last succeeded DP
bool print_yr_; // YR:i: # redundant seed hits
bool print_zf_; // ZF:i: # FM Index ops
bool print_zi_; // ZI:i: # extend loop iters
bool print_zp_; // ZP:i: Score of best/second-best paired-end alignment
bool print_zu_; // ZU:i: Score of best/second-best unpaired alignment