Do you have a picture? Some single ones are used to increase or decrease scores in which you win a game.

There are 3 plugs lower middle

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I could not find top card, as an example I found Captain Card.

Not sure what that plug is for F/S unit?

If you look at the right most side of the relays , there seems to be an open spot.. the F/S unit generally has 2 connectors.

There should be an F relay (AS mini stepper style) next to the L relay in the relay rack. The two connectors are for the F relay.

/Mark

It's missing. On this game the F relay drives the I/Alternating relay during the game and the match circuitry at the end of the game.

As Mark notes, since you have no F relay to drive your I relay, your game will not be toggling the I alternating features such as lighting the pop bumper lights and associated elevated scoring or moving the lit special(s) to the other location(s).

And also you will not get any match lights or awarded credits for matches at end of game.

No, the I relay is a separate unit elsewhere that is energized whenever F pulses.

EDIT: actually, one pulse to the F relay causes the I relay to pull in (and remain pulled in), the next pulse to F causes I to release. This cycle continues with each pulse of F.

No, the I or Alternating relay is probably in the main cabinet, either on the bottom board or under the playfield. The F relay that you're missing is just a single AS style relay with a small ratcheting mechanism, a pair of small circuit boards and a single normally open switch that connects to the rest of the game through a pair of jones plugs. There are probably pictures of AS relays elsewhere on the forum.

While many AS relays look similar there may be differences from game to game. They could be configured with various cams and switches. So unless you can find another F Relay from a Top Card, you'll want to compare the schematics of Top Card against whatever the donor game the replacement comes from, or photos of the two relays.

According to the Top Card schematic, the missing 00-90 Unit relay (F) has 1 switch that drives the Alternating relay (I), and 2 disks. One disk lights the match number at the end of game (schematic 5B), and the other disk completes the circuit to award a replay if the player matches (schematic 9.5E)

To build one from scratch, you're going to have to start with these parts or a unit like it.
pbresource says they're out of stock on some of the parts so you'll probably need to start with a used one.
http://www.pbresource.com/relay-GTB.html#gtb-as

Let me know if I can help.

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It's kind of hard to adjust (some say almost impossible) so having it on a workbench makes working on it a lot easier.