to be to [VERB]

It's also used to communicate a command from someone else. For instance, here the speaker is communicating a command given by the boss. He wants them to stay until the report is finished. No exceptions!

The boss says we're to stay here and work on this report until it's finished — even if it takes all night!

It can even be used in the passive. Say you're an investigator at a crime scene directing your team of investigators:

I want pictures of everything, but nothing is to be touched or moved!

In other words, this phrase is used to express that something ABSOLUTELY AND CATEGORICALLY MUST (NOT) be done, so use it wisely or you may sound bossier than you mean to. For instance, when addressing your roommates. The following would only be appropriate to say if your roommates have a history of eating other people's food: 

There's a cake in the refrigerator. It's for my niece's birthday party tomorrow. None of you is to eat it!

Here, the trainer could've also said If I'm GOING TO be... with the same meaning. Using either of these phrases makes it sound more or less like the trainer has already agreed to take the job, but is very serious about his conditions. 

In this usage, the if is sometimes strongly emphasized. For instance, in the following example, the speaker has more or less decided to hire you, but again, under one strong condition:

If you're to work for me, you cannot — under any circumstances — smoke cigarettes. I can't stand the smell.

Let me give you another example. Say you have a catering company, i.e. you provide food and servers for parties and events. You're asked to potentially provide catering for a huge event with over 1000 people at some future point. They still haven't set the date, but they want you to give them a price quote, so they can come up with a budget. You give them a quote and propose a menu, and it's accepted. But you have one condition: 

If my company is to cater this event, we're going to need at least two weeks' notice. 

This implies that you've already more or less accepted the job, but the only way you'll commit fully is under the condition that they let you know the actual date of the event at least two weeks in advance. 

However, let's say you weren't impressed with their offer and wanted to sound more detached and hypothetical. In that case, instead of if my company is to... you'd simply keep things more vague by using the subjunctive: if my company were to cater this event... This would make it clear that you haven't decided one way or the other and everything is still under discussion; up in the air.