from the NHL website,

Unsigned veteran players can earn a contract with an NHL team by impressing enough in training camp on a professional tryout contract (PTO). Training camps open Wednesday and the regular season begins Oct. 10.

The following notable players (listed alphabetically) will be in an NHL camp on a PTO:

Zach Aston-Reese, Carolina Hurricanes

The 29-year-old forward had 14 points (10 goals, four assists) in 77 regular-season games with the Toronto Maple Leafs last season, and one goal in six Stanley Cup Playoff games. Aston-Reese has 80 points (42 goals, 38 assists) in 307 regular-season games with the Maple Leafs, Anaheim Ducks and Pittsburgh Penguins and five points (two goals, three assists) in 29 playoff games.

Josh Bailey, Ottawa Senators

The 33-year-old forward had 25 points (eight goals, 17 assists) in 64 games with the New York Islanders last season. Bailey has 580 points (184 goals, 396 assists) in 1,057 regular-season games during 15 seasons, all with the Islanders, and 50 points (16 goals, 34 assists) in 71 playoff games.

continue for many more names..