With just two weeks to go until Election Day, Americans can’t turn on the television without being inundated with political ads.

But behind those ads lies a complicated strategy game, what Republican operative Brian O. Walsh likes to think of as three-dimensional chess.

The movement of ad reservations signals a lot about which House and Senate races are most competitive and which party has the upper hand. Democrats are currently trying to expand the House playing field by putting money into newly competitive districts. Congressional Leadership Fund, the super PAC tied to House GOP leadership, is reserving time in second-tier races as an insurance policy against a Democratic wave.

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Party committees are also pulling out of races. In Florida and Ohio, for example, the Democratic Senatorial Campaign Committee has cut its reservations in Senate races that look like steeper climbs than once thought for Democrats. 

But with any ad buy or cancellation, there’s often more than meets the eye. So here are six basics to remember about reservations:

1. Rates vary  

Candidates pay a lower rate for TV time than do party committees or super PACs.

Rates are also highly dependent on the programming. Buying a slot during a daytime talk show, for example, is cheaper than prime time.

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Rates also depend on the media market and the demand for time there. As with a commodity, the rules of supply and demand play a big role in the pricing of ad time (what ad buyers call “inventory”). These differences determine how much bang for the buck a political advertiser gets. House Majority PAC, for example, has been a big spender in both Maine’s 2nd District and New Jersey’s 5th District, but the super PAC’s dollars go further in Maine, where advertising is cheaper than in the New York City media market. Prices have skyrocketed in markets that are Senate and presidential battlegrounds like Nevada and North Carolina.

[House Majority PAC Reserves TV Time Early for Fall]

2. Reservations save money (in theory)

Ad rates get pricier closer to the election. So the benefit of reserving early is the ability to lock in time for later at a lower rate. House Majority PAC, for example, saved $13 million in 2014 by reserving time early. They’ve tried to do the same this year by making their first fall ad reservations in March. 

Candidates who reserve don’t always end up paying what they bargained for, though. That’s because as more groups and candidates enter the market, the price of inventory goes up. In order to keep their time, the campaigns have to pay the going rate. Flexible campaigns, said GOP media consultant Ben Burger, build this into their budgets.

3. Nothing is permanent 

A reservation is not a buy. (And even when the media talks about groups making a “buy,” that often doesn’t mean the groups have already laid down the cash.) Any reservation can be canceled until two weeks before the ads are supposed to start. Campaigns, committees and super PACs shift reservations around in the final weeks once they have updated polling data telling them whom to target. 

[Hybrid Ads Allow Parties and Campaigns to Save Money, but at What Cost?]

4. Many reasons to shift

The party committees and super PACs typically pull out of a market for two reasons. Either they feel confident (as Democrats do in New Hampshire’s 1st District) or they don’t see a path to victory. House Majority PAC pulled out of Minnesota’s 3rd District, which was seen as a sign that the race was tougher for Democrats. HMP could still rebook time there.

“It’s the party’s own committee saying, ‘You’re the loser,’” said one Democratic consultant of any outside group’s decision to cut spending in a challenging race.

[GOP Super PAC Doubles Its 2014 Spending]

Committees and super PACs also add reservations late, sometimes using money they’ve diverted from elsewhere. The Democratic Congressional Campaign Committee, for example, recently reserved time in Kansas’ 3rd District — a typically red suburban district Democrats are hoping Donald Trump’s sagging poll numbers will help them pick off this year.

HMP and — and now the National Republican Congressional Committee — went up late in Indiana’s 9th District, where Democrats think they have a chance this year. The NRCC also plans to spend soon in Florida’s 7th District, where Democrats are already spending big. GOP Rep. John Mica didn’t make Roll Call’s most vulnerable list until late this year.

5. Reservations are a signaling game

Campaigns, party committees and super PACs can’t legally communicate. But they naturally want to avoid running ads for the same candidate in the same market during the same week. That would be inefficient. So they track each other’s media buys to know when other players are going on air.

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The opposing side tracks all those buys, too. So sometimes one side will go into a district just to get the other side to spend there, drawing down the resources they have to spend elsewhere. 

6. The buyers are invaluable 

Ad buying is both a science and an art, consultants agree.

“Candidates always want to meet the media consultants,” said GOP operative Bob Kish. “But they should also meet the media buyers because they’re spending 90 percent of your money.” 

Knowing how to negotiate rates and knowing how certain media markets behave takes experience.

“It’s like any war,” Burger said. “The more times you go out on the battleground, the better soldier you’ll be.”