When sending a campaign, you are able to use a number of custom tags and filters.
Most common tags are listed below:

Now, each of the above tags is able to receive a set of filters.
Filters are a simple way of transforming the tag in a way or another, for example you might want to embed a sharing link to twitter in your campaign, say the campaign url itself.

Using only the tags you would embed it like:

https://twitter.com/intent/tweet?text=[CAMPAIGN_SUBJECT]&url=[CAMPAIGN_URL]

But there is a problem, because twitter expects your arguments to be url encoded, and by that, i mean twitter expects to get

https://twitter.com/intent/tweet?text=my%20super%20campaign&url=http%3A%2F%2Fwww.domain.com%2Fcampaigns%2F1cart129djat3

but instead it will get https://twitter.com/intent/tweet?text=my super campaign&url=http://www.domain.com/campaigns/1cart129djat3

In order to overcome this issue, we will apply filters over our tags, therefore, the twitter url becomes:

https://twitter.com/intent/tweet?text=[CAMPAIGN_SUBJECT:filter:urlencode]&url=[CAMPAIGN_URL:filter:urlencode]

Pretty simple eh?
But we can do even more, let's say we want to make sure our twitter text starts with a capitalized letter and the rest of the letters will be lowercase.
In order to accomplish this, we can apply multiple filters(separate by a pipe) to same tag, for example:

https://twitter.com/intent/tweet?text=[CAMPAIGN_SUBJECT:filter:lowercase|ucfirst|urlencode]&url=[CAMPAIGN_URL:filter:urlencode]
Please note, the order in which you add the filters is the same order they are applied.

Bellow is the entire list of filters, for now there are a few, but in the future the number might increase.