Ok, so i messaged BI Directly and was provided with a link to this page.
So DEVs or community. A lot of people ask that slots can be reserved for specific player steam id's but I've thought long and hard about this and think a better solution would be password enabling / disabling option in the lobby by admin or host, allowing a player to keep a slot open should they enjoy a server and wish to continue with this player slot and save his / her progress.
This way there is much more freedom and flexibility within a server allowing a simplistic way of reserving a slot without losing your game progress.
Ideal for a long term survival server. Maybe have the player character continue to use up their thirst and hunger whilst player is away from server leaving their character in a sleep state within a building, tent or round a camp fire helping them to keep warm and maintain their energy using up less resources.
When the player returns to their slot the unit will come out of sleep state and the player can continue where they left off, providing they haven't been mauled in their sleep by a zombie or taken captive by the enemy unknown to them.
So this is why we need passworded slots!
Thanks guys and hopefully Arma Devs can make this happen. If not I'm sure someone in the Arma modding community can pul this off.