Control

In generative syntax, control is the phenomenon that the understood (PRO) subject of an infinitival clause must be anaphorically dependent on a specific argument of the matrix clause.

In phonology, control is a mechanism introduced in DiSciullo & Williams (1987) which assigns a special linkage between the argument of an affix and a position in the verb's argument structure, thereby preventing that argument from being realized syntactically.

Examples
In (i), the understood subject of to leave must be John; in (ii), it is Susan. The case in (i) is called 'subject control' (the so-called controller John is a subject), the case in (ii) 'object control' (Susan is an indirect object).

(i) John promised Susan to leave

(ii) John persuaded Susan to leave

The argument structure of the English verb employ consists of two arguments (viz. AGENT and THEME). If the suffix -er is added to this verb, the resulting noun employer is an AGENT, and in John's employer, John cannot realize that argument. On the other hand, if the suffix -ee is added to this verb the resulting noun employee is a THEME, and in the NP John's employee, the noun John cannot express the THEME role. DiSciullo & Williams account for the difference between -er and -ee by assuming that the suffix -er controls the external argument (= AGENT role) of the verb employ, while -ee controls its internal argument (= THEME role). In both cases, the controlled argument cannot be expressed in syntax.

Link
Utrecht Lexicon of Linguistics