Because the letter "S" begins with a vowel sound, when pronouncing the abbreviation SLA as individual letters, an SLA is correct.
The letter "S" begins with a vowel sound; therefore, the abbreviation SLA takes the indefinite article "an" (not "a").
An SLA defines the level of service expected by a customer from a service provider.
By contrast, when pronouncing or writing this expression as words (service-level agreement), the first word (service) begins with a consonant sound, which takes the article "a".
A service-level agreement is a contract between a supplier and its customers.
Be aware that the choice between "a" and "an" is determined by the pronunciation of the word that follows, not by its spelling.
An SLA should include a description of the services to be provided as well as metrics by which the services are measured.
Thus, when referring to the single letter "S", we say an S (not "a S").
The name of my best friend begins with an R.
This is a partial list of abbreviations that also begin with the letter "S" and are typically pronounced as separate letters. Consequently, they take the article "an" (not "a"):