Since this topic has come up a couple of times in the comments of this blog, here is my attempt to explain the differences between these designations (subject to change, of course). Alison and I exchanged comments in The Entry Level/Untenured Bid List is Out, if you prefer that format.
The best source for up-to-date information on a post's differential and allowances is the Department of State's Office of Allowances in the Bureau of Administration. They are the keepers of these and the Foreign Per Diem Rates.
They publish the latest schedules on their website, divided into:
Post Cost of Living Allowance (COLA)
Post (Hardship) Differential
Living Quarters Allowance
Danger Pay Allowance
Foreign Per Diem Rates
Their definitions for each of these is listed in their Summary of Allowances and Benefits.
You can also look at all of these by post with their Allowances by Location section. You can find this information through other sources during A-100, but it comes in very handy for biding on subsequent tours.
The first two of these four designations are pretty straight-forward (from the website):
"The Post Hardship Differential is meant to compensate employees for service at places in foreign areas where conditions of environment differ substantially from conditions of environment in the continental United States and warrant additional compensation as a recruitment and retention incentive. It is paid as a percentage of basic compensation in 5, 10, 15, 20, 25, 30 and 35% increments." To see which posts have a Hardship Differential just go to Post (Hardship) Differential and look down the list.
"The Danger Pay Allowance provides additional compensation for employees serving at designated danger pay posts. It is paid as a percentage of basic compensation in 15, 20, 25, 30 and 35% increments." To see which posts have a Danger Pay Allowance, just go to Danger Pay Allowance and look own the list.
Now the Service-Needs Differential and Hard-to-Fill designations are trickier. If you are in A-100 or an untenured officer, only the Hard-to-Fill designation applies, and only for your second tour bidding.
For example in our case, the Entry Level Position in Kolkata was designated as a Hard-to-Fill job at post. The benefit we received from this was an extra 5% differential during our second tour bid. The standard differential (hardship plus danger) for Kolkata was 25%, plus the extra 5%, gave us 30% differential for bidding purposes. And as I mentioned in The Entry Level/Untenured Bid List is Out this gave us a nice boost in the assignment order.
A Hard-to-Fill job at a post is defined as when less than two officers in grade and at cone bid on a specific job at that post. This typically refers to tenured officer bidding. State's Human Resources maintains a list of Hard-to-Fill jobs at posts. Your Career Development Officer is also a good source for this information, if you are in A-100 or an untenured officer.
Next up, Service-Needs Differential:
A post is designated as a Service-Needs Differential post when it is Historically-Difficult-to-Staff.
If you are a tenured officer and serving at a post qualifies you for a Service-Needs Differential, you currently get a 15% of your salary bonus for three years, if you extend your two year posting to three years. You have up to six months after your arrival at post to decide on taking the opportunity. If you curtail or leave post early, you have to return the 15%.
For untenured officers, since the first two tours are only two years each, you cannot volunteer to extend to get the Service-Needs Differential at a Historically-Difficult-to-Staff post. A Hard-to-Fill job may qualify for the Service-Needs Differential if it is in a Historically-Difficult-to-Staff post.
Here is the chain of events that leads a post to become designated as Service-Needs Differential:
1. Hard-to-Fill – when less than two officers in grade and at cone bid on a specific job at post
2. Most-Difficult-to-Staff – when more than half of the jobs available at a post where Hard-to-Fill
3. Historically-Difficult-to-Staff – when a post has been Most-Difficult-to-Staff for the 3 out of the last 4 years
That may of been too much information, but may be helpful at a later time. Please add to the above with comments!
Update: I added the tags "trailing partner" and "preparation to move" since the comments generated discussion on those topics.