For limite companies the guarantor is nominated by the applicant during the application process and personal guarantee email invite is sent to the guarantor so they can either accept or decline the personal guarantee request. 

The guarantor is given a view access to the application and the credit limit requested and is then asked to fill in some of their personal details ( full name, date of birth) and accept the personal guarantee terms & conditions.

If the applicant is also the director or shareholder - they can just tick a box during the application to accept or decline the guarantee.

Sole traders, trustees of an unincorporated trust, partners in a partnership &  personal applicants are automatically liable & do not need to separately sign a personal guarantee